The selective catalytic reduction of NO_x over a Cu/ZSM-5/SAPO-34 composite catalyst

Abstract

A series of Cu/ZSM-5/SAPO-34 (Cu/ZS-PM) composite catalysts with fixed Cu amount and varying ZSM-5 mass fraction were synthesized using a pre-seed method, and their catalytic performances were tested for selective catalytic reduction (SCR) of NO with NH₃. The catalysts were characterized by means of XRD, FT-IR, SEM, BET, UV-vis DRS, NH₃-TPD, H₂-TPR and in situ DRIFTS. The results indicate that the high deNO_x performance of the Cu/ZS-PM-15% catalyst was due to the excellent redox property and the formation of NO₂. The acidity of the catalysts was significantly influenced by the content of ZSM-5 seeds, whereas there was no obvious correlation between acid strength and NH₃-SCR performance. The in situ DRIFTS spectra demonstrated that both Lewis and Brønsted acid sites are involved in the NH₃-SCR reaction and the reaction on Brønsted acid sites is more active than that on Lewis acid sites. NO₂ is a primary intermediate following NO adsorption on Cu/ZS-PM catalysts, which may promote a “fast” SCR reaction at low temperature.

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1. Introduction

Nitrogen oxides (NO_x), which result from automobile exhaust gases and industrial combustion of fossil fuels, are a major source of air pollution and can cause a series of environmental issues, such as photochemical smog and fine particle pollution (haze).¹ Nowadays, regulations confining the emission of NO_x from diesel engines are becoming more and more stringent.² Numerous approaches have been proposed for lean-NO_x abatement, each of them with its own specific sets of problems. The selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) is one of the most promising technologies for NO_x emission control.^3,4 Transition metal (in particular Fe and Cu) ion-exchanged zeolite catalysts have shown high activity and N₂ selectivity.

The most extensive studies have been carried out on Cu²⁺ ion exchanged ZSM-5 (Cu/ZSM-5) zeolites, which firstly showed high NO decomposition rates and NO_x-SCR activities in the 1980s.^5–9 Nevertheless, the poor hydrothermal stability is still a primary concern in its commercial application to the diesel engine after-treatment system,^10,11 limiting its further use in the SCR processes. Recently, Cu²⁺-exchanged CHA zeolite, such as Cu/SAPO-34 has been found to exhibit higher activity in NH₃-SCR reaction and better hydrothermal stability than Cu/ZSM-5 catalysts.¹² However, it was reported by Epling et al.¹³ that kinetical limitation at low reaction temperature for NH₃-SCR of NO_x was remarkable over Cu/SAPO-34 catalysts due to their small pore sizes, which greatly restricted the access of reactants to the zeolite active sites.

Micro–mesoporous composite materials have been reported to exhibit excellently catalytic performances due to the reduction of diffusion resistance, and many composite materials were synthesized and applied in different catalytic processes.^14–22 Li and coworkers²³ synthesized ZSM-5 seeds-grafted SBA-15 composite as a support for cobalt Fischer–Tropsch synthesis catalysts and found that Co/SBA-15/ZSM-5 composite catalyst showed a higher conversion than the Co/SBA-15 due to the improvement of acidity derived from ZSM-5. However, the poor hydrothermal stability of the mesoporous materials is not suitable for higher temperature reaction. Inspired by this study, ZSM-5, possessing a larger pore diameter which can reduce the diffusion resistance, and SAPO-34 with a highly hydrothermal stability were combined. A series of ZSM-5/SAPO-34 composite materials were successfully synthesized by a pres-seed method. The corresponding catalysts Cu/ZSM-5/SAPO-34 were evaluated in NO_x SCR reaction with NH₃ as the reductant. The relationship between the structural characteristics of Cu/ZSM-5/SAPO-34 catalysts and NH₃-SCR performance was also investigated in this work.

2. Experimental

2.1. Preparation of supports

2.1.1. Synthesis of nano-size SAPO-34 zeolite. Nano-size SAPO-34 zeolite was prepared as follows: 0.50 g boehmite and 0.22 g deionized water were mixed together and stirred for 0.5 h at room temperature. And then 4.05 g tetraethyl ammonium hydroxide (TEAOH), 0.79 g phosphoric acid aqueous and 0.31 g silica sol were added successively and stirred vigorously until a homogeneous solution. The composition of the resulting homogeneous gel was 0.6 SiO₂: 1.0 Al₂O₃: 0.5 TEAOH: 1.0 P₂O₅. Subsequently the gel was transferred to a Teflon autoclave and maintained at 200 °C for 48 h. The solid product was filtered, washed, dried at 100 °C for 12 h, and calcined at 550 °C for 6 h to remove the template. Finally, the nano-size SAPO-34 zeolite was obtained.

2.1.2. Synthesis of ZSM-5/SAPO-34. For synthesis of series ZSM-5/SAPO-34 composites, ZSM-5 seeds solution was prepared from synthetic gel composition 1.0 Al₂O₃: 80 SiO₂: 16 TPABr: 300H₂O. Hydrothermal crystallization of the synthetic gel was carried out at 120 °C for 32 h in Teflon autoclave. Aluminum isopropoxide (Yakuri Pure Chem. 98%) and tetraethyl orthosilicate (Sigma-Aldrich, 99%) were used as sources of Al and Si, respectively. TPABr (tetra-propyl ammonium bromide, Sigma-Aldrich, 98%) was also used for the formation of ZSM-5 structure.

Nano ZSM-5/SAPO-34 molecular sieve prepared by pre-seed method was obtained by a one step crystallization of SAPO-34 synthetic gel. Detailed procedure was as follows: 24.27 g TEAOH was mixed with 1.32 g deionized water and stirred for 1 h. 3.0 g boehmite, 4.75 g phosphoric acid aqueous and 1.86 g silica sol were added successively and continued to stir for 10 h, and then different amounts of ZSM-5 seeds solution was added to the above solution. The resulting reaction mixture was stirred at room temperature for 30 min and then transferred to a Teflon autoclave for hydrothermally treatment at 200 °C for another 72 h. As-synthesized composites were obtained by filtration, washing and drying at 100 °C for 12 h. Finally, calcination was carried out at 550 °C for 6 h to remove template. The resulting ZSM-5/SAPO-34 composite zeolite denoted as ZS-PM-x (x = 15%, 25%, 35% and 50%, where x represents the mass fraction of ZSM-5 zeolite in the composite).

For comparison purpose, ZS-MM-15% was also synthesized, which was a mechanical mixture of ZSM-5 and SAPO-34 zeolite. Where, 15% represents the mass fraction of ZSM-5 zeolite in the ZS-MM.

2.1.3. Preparation of the catalysts. Cu/SAPO-34, Cu/ZSM-5, Cu/ZS-PM-x and Cu/ZS-MM-15% catalysts with Cu loading of 4.0 wt% were prepared by incipient-wetness impregnation method.²⁴

2.2. Characterization of the catalysts

X-ray diffraction (XRD) patterns of the samples were performed on a Bruker D8 Advance diffractometer using Cu Kα (λ = 0.15406 nm) radiation with a Nickel filter operating at 40 kV in the 2θ range of 5–50°. Fourier transform infrared (FT-IR) absorbance spectra were recorded in the wave numbers ranging from 400 to 1600 cm⁻¹ on a FTS-3000 spectrophotometer. Scanning electron microscopy (SEM) measurement was conducted on a Quanta 200F instruments operating at 20 kV. Samples were dusted on an adhesive conductive carbon belt attached to a copper disk and were coated with Au prior to measurement. N₂ adsorption–desorption isotherm was measured using a Micromeritics TriStar II 3020 porosimetry analyzer at 77 K. Diffuse reflectance ultraviolet-visible (UV-vis DRS) spectra were recorded on a UV-vis spectrophotometer (Hitachi U-4100) in the range 200–800 nm and with dehydrated BaSO₄ as the internal standard reference. The acid strength of the support and catalysts were determined by NH₃-TPD measurements performing on a home-built apparatus. Temperature programmed reduction (TPR) experiments were performed in a home-made apparatus. Prior to reduction, the samples (50 mg) were treated from room temperature to 600 °C under a flow rate of 30 ml min⁻¹ N₂ and then cooled down to 100 °C in a purging N₂ flow. The temperature programmed reduction was carried out in 30 ml min⁻¹ flow of 10% H₂/N₂ at a ramp rate of 10 °C min⁻¹ up to 600 °C. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments were recorded on an FTIR spectrometer (Nicolet NEXUS 6700) equipped with a smart collector and an MCT/A detector. Prior to each experiment, the sample was purged in flowing N₂ at 500 °C for 1 h.

2.3. NH₃-SCR activity evaluation

NH₃-SCR activity measurement was carried out in a fixed-bed quartz micro-reactor at atmospheric pressure. 0.4 g of catalyst with 40–60 mesh particle size was used in each test. The reactant gas compositions were as follows: 1000 ppm NO, 1000 ppm NH₃, 3 vol% O₂ and balance N₂. The total flow rate was 500 ml min⁻¹ and thus a GHSV of 50 000 h⁻¹ was obtained. The temperature varied from 100 to 500 °C, and the concentration of NO in the inlet and outlet gas was analyzed by a NEXUS 670-FTIR spectrometer equipped with a multiple-path gas cell (2 m). The NO conversion and N₂ selectivity were calculated as follows:

3. Results and discussion

3.1. NH₃-SCR performance

A series of Cu/ZS-PM-x catalysts supporting 4 wt% Cu with different amounts of ZSM-5 are investigated in the NH₃-SCR of NO and the results are shown in Fig. 1a. The activity of Cu/ZS-MM-15% is also investigated for comparison. NO conversion changes with the increasing of reaction temperature over all the samples. For Cu/ZSM-5 catalyst, NO conversion above 90% is only from 225 to 350 °C. Cu/ZS-PM-15% catalyst exhibits the best activity with NO conversion above 90% from 150 to 450 °C. Cu/SAPO-34 sample shows lower activity with NO conversion above 90% from 175 to 450 °C. Moreover, with the increasing of the mass fraction of ZSM-5 in Cu/ZS-PM catalysts from 15% to 50%, NO conversion dramatically decreases from 92.4% to 60.4% at 150 °C. Compared with Cu/ZS-PM-x catalysts prepared by pre-seed method, Cu/ZS-MM-15% catalyst synthesized through mechanical mixture method shows a relatively higher low-temperature NO conversion and a much lower NO conversion at high temperature (>350 °C). As displayed in Fig. 1b, N₂ selectivity was above 90% for all of the catalysts in the test temperature range. In addition, Fig. 1c shows the NO conversion of Cu/ZS-PM-15% and Cu/ZS-MM-15% catalysts with time on stream at 350 °C. Cu/ZS-PM-15% composite catalyst exhibits a much higher NH₃-SCR activity than Cu/ZS-MM-15% in the range of 72 h, indicating that the composite catalyst is beneficial to the improvement of stability. The results of activity test suggest that a synergistic effect should originate from the composite of ZSM-5 to SAPO-34. The following characterization of catalysts will supply more information about the relationship between the structure of catalysts and NH₃-SCR activity.

Fig. 1 NO conversion (a), N₂ selectivity (b) as a function of reaction temperature over Cu/SAPO-34, Cu/ZSM-5, Cu/ZS-PM-x with variable ZSM-5 amounts and Cu/ZS-MM-15% catalysts in the temperatures ranges of 100–500 °C and catalytic activities of Cu/ZS-PM-15% and Cu/ZS-MM-15% catalysts with time on stream (c).

3.2. XRD patterns

The powder XRD patterns of SAPO-34, ZSM-5, ZS-PM-x composite materials and ZS-MM-15% are shown in Fig. 2A. The diffraction peaks at 2θ = 7.94°, 8.85°, 14.90°, 23.33°, 23.90° and 24.50° can be ascribed to the characteristic peaks of ZSM-5 zeolite,⁴ and 2θ = 9.59°, 20.84°, 30.73° and 31.22° are assigned to SAPO-34 zeolite.²⁵ The peak intensities are not very strong due to the presence of nano-size of ZSM-5 phase in the composite material.¹⁷ ZS-MM-15% sample also displays similar diffraction peaks but with a relatively stronger intensity due to the presence of individual crystals of ZSM-5 and SAPO-34. The result indicates the co-existence of ZSM-5 and SAPO-34 zeolite phases in the as-synthesized composite samples, and no other miscellaneous peaks are observed. It reveals that binary ZSM-5/SAPO-34 composite might be successfully prepared through the pre-seed method.

Fig. 2 XRD patterns of supports: (A) SAPO-34, ZSM-5, ZS-PM-x and ZS-MM-15% and catalysts (B) Cu/SAPO-34, Cu/ZSM-5, Cu/ZS-PM-x and Cu/ZS-MM-15%.

XRD patterns of all catalysts displayed in Fig. 2B exhibit the same characteristics as their corresponding supports, which indicates that the binary structure were retained after the impregnation of the active metal. Furthermore, the crystalline phases are not detected in Cu/SAPO-34, Cu/ZSM-5, Cu/ZS-PM-x and Cu/ZS-MM-15% catalysts,²⁶ indicating that Cu species are homogeneously dispersed on the supports and the particle size is so small that it is below the detection limit of the X-ray signals.

3.3. FT-IR analysis

IR spectroscopy, which is sensitive to the ordering of silicate structures at the molecular scale, is often used primarily to examine the organization of an ordered zeolitic network at a scale as low as a few unit cells.²⁷ The FT-IR spectra of ZSM-5, SAPO-34, ZS-PM-x composites and ZS-MM-15% in the region of 400–1600 cm⁻¹ are depicted in Fig. 3. By identifying the IR bands, one can get some useful information on the framework structure of molecular sieve. All of the composites (ZS-PM-x) and mechanical mixture (ZS-MM-15%) presented the characteristic IR bands attributed to SAPO-34 material at 636 cm⁻¹ (six-member rings of T–O–T, T = Si or Al) and 722 cm⁻¹ (symmetric stretch of P–O groups),^28–30 and ascribed to ZSM-5 material at 453 cm⁻¹ (T–O band, T = Si or Al), 548 cm⁻¹ (five-member rings of T-O-T), 797 cm⁻¹ (asymmetric stretch of T–O–T) and 1224 cm⁻¹ (T–O band).^17,31 Additionally, for ZS-PM-x samples, the intensity of the bands assigned to ZSM-5 material becomes stronger with the increasing of the content of ZSM-5 seeds. Compared with ZS-PM-15% composite, the mechanical mixture (ZS-MM-15%) exhibits relatively strong characteristic peaks. FT-IR spectra results indicate that ZS-PM-x composites contain the primary units of both ZSM-5 and SAPO-34, and it is notably different from the mechanical mixture. Combining with the results of XRD and FT-IR, it is reasonable to conclude that binary ZSM-5/SAPO-34 composite has been successfully prepared through the pre-seed method, which should be beneficial to reducing diffuse resistance and then enhance NH₃-SCR performance.

Fig. 3 FT-IR absorbance spectra of supports: SAPO-34, ZSM-5, ZS-PM-x and ZS-MM-15%.

3.4. SEM observations

The SEM images of SAPO-34, ZS-PM-x composites, ZS-MM-15% samples and EDS elemental mapping of the copper species on Cu/ZS-PM-15% are shown in Fig. 4. It is noted that the as-synthesized nanocrystalline SAPO-34 zeolite possesses an average particle size around 500 nm. When ZSM-5 seeds were introduced into SAPO-34 synthetic gel, a notable differences can be noticed that the cube SAPO-34 zeolite is partly enwrapped by some small ZSM-5 crystals with size about 50–250 nm, and the coverage density becomes larger with the increasing of ZSM-5 seed amount. As ZSM-5 content increases to 50 wt%, the nanocrystalline SAPO-34 zeolite are not readily observed, since the cube SAPO-34 crystals were enwrapped completely by the nano-size ZSM-5. It might be a reason why ZS-PM-x composites show weak SAPO-34 characteristic peaks in XRD and FT-IR results.³¹ In addition, compared with ZS-PM-x composites, SEM image of mechanical mixture (Fig. 4f) clearly shows two kinds of individual crystals. The larger crystals are assigned to SAPO-34 and the smaller crystals due to ZSM-5. It is obvious that the interaction among them cannot be found. A higher magnification SEM image for Cu/ZS-PM-15% sample together with O, Al, Si, P and Cu elemental mapping is conducted and the result is shown in Fig. 4g. It is clear that copper species are highly dispersed on Cu/ZS-PM-15% catalyst.

Fig. 4 SEM images of SAPO-34 (a); ZS-PM-15% (b); ZS-PM-25 (c); ZS-PM-35% (d); ZS-PM-50% (e); ZS-MM-15% (f) and EDS elemental mapping of Cu/ZS-PM-15% catalyst (g).

3.5. N₂ adsorption–desorption characterization

The porous properties of ZSM-5, SAPO-34, ZS-PM-x composites and ZS-MM samples were examined by nitrogen adsorption–desorption isotherms at 77 K and the results are displayed in Fig. 5A. All supports exhibit typical type-I isotherms characteristic of microporous materials.³² Furthermore, the shape of N₂ sorption isotherms for ZS-PM-25%, ZS-PM-35% and ZS-PM-50% changes with the increasing of the amounts of ZSM-5 seed. The N₂ sorption isotherms of supported catalysts are shown in Fig. 5B. It can be seen that all catalysts still keep type-I isotherm, indicating the loading of active components did not change the pore structure of supports.

Fig. 5 Nitrogen adsorption–desorption isotherms of supports: (A) SAPO-34, ZSM-5, ZS-PM-x and ZS-MM-15% and catalysts (B) Cu/SAPO-34, Cu/ZSM-5, Cu/ZS-PM-x and Cu/ZS-MM-15% catalysts.

The textural and structural properties of the samples are summarized in Table 1. Both specific surface area and pore volume of support increase with the increasing of the mass fraction of ZSM-5 in ZS-PM-x composites, and the mechanical mixture ZS-MM-15% sample possesses a lower specific area than ZS-PM-15%. Moreover, they are less than those of samples before loading due to the deposition of metal oxides on the inner surface of pore channels.

Table 1 Textural properties of H-ZSM-5, H-SAPO-34, ZS-PM-x, ZS-MM-15% supports and their corresponding supported Cu catalysts

Samples	SBET^a (m^2 g^−1)	SMic^a (m^2 g^−1)	Vt^b (ml g^−1)
a Calculated by BET method.b Calculated by t-plot method.
H-ZSM-5	348	246	0.097
H-SAPO-34	567	543	0.063
ZS-PM-15%	536	499	0.072
ZS-PM-25%	528	497	0.073
ZS-PM-35%	492	451	0.084
ZS-PM-50%	453	398	0.101
ZS-MM-15%	495	465	0.076
Cu/ZSM-5	262	168	0.090
Cu/SAPO-34	419	391	0.062
Cu/ZS-PM-15%	426	393	0.065
Cu/ZS-PM-25%	421	389	0.070
Cu/ZS-PM-35%	357	318	0.075
Cu/ZS-PM-50%	325	281	0.092
Cu/ZS-MM-15%	301	269	0.069

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3.6. NH₃-TPD results

The catalyst surface acidity plays an important role in NH₃-SCR reaction. The NH₃-TPD profiles of pure H-SAPO-34, Cu/SAPO-34, Cu/ZS-PM-x and Cu/ZS-MM-15% samples are displayed in Fig. 6 and the peak area data of NH₃ consumption are listed in Table 2. Two distinctly resolved NH₃ desorption peaks centered at around 226 and 520 °C are observed for parent H-SAPO-34, corresponding to the weak and the strong acid sites, respectively.^33,34 The first peak can be attributed to physisorbed NH₃ or ammonium species adsorbed at the weak acid sites, while the second one above 400 °C can be ascribed to NH₃ strongly adsorbed at the strong acid sites. For Cu/SAPO-34 and Cu/ZS-PM-x catalysts, the intensity of the desorption peak, especially the higher peak, is much lower than that of H-SAPO-34 due to that the Brønsted acid protons were substituted by Cu²⁺.^35,36 Moreover, the peak temperatures of all samples obviously shifted towards a lower temperature than that of H-SAPO-34 support, indicating a decrease in the acid site density. Furthermore, the intensity of the lower peak of Cu/ZS-PM-x catalysts decreased with the increasing of the content of ZSM-5 seeds, while the higher peak assigned to NH₃ adsorbed on strong acid sites almost remain unchanged (as shown in Table 2). This gradual temperature variation for Cu/ZS-PM-x catalysts may be due to the fact that the doping of variable ZSM-5 results in the change of the amount of active species, and then causing the change of catalyst acidity. Compared with Cu/SAPO-34 and Cu/ZS-PM-x catalysts, Cu/ZS-MM-15% exhibits much lower acidity. Although Cu/ZS-PM-15% catalyst shows the best activity, its acid strength is less than that of Cu/SAPO-34 catalyst. Therefore, it can be conclude that there are no obvious correlations between acid strength and NH₃-SCR performance. The proper amount of surface acidity appears to be crucial for the excellent catalytic performance. This result is in consistent with the finding of Li et al.³⁷

Fig. 6 Temperature-programmed desorption of ammonia of H-SAPO-34, Cu/SAPO-34, Cu/ZS-PM-x and Cu/ZS-MM-15%.

Table 2 Acid properties of H-SAPO-34, Cu/SAPO-34, Cu/ZS-PM-x and Cu/ZS-MM-15% catalysts

Sample	Peak area^a		Total area
	Peak 1 (∼200 °C)	Peak 2 (>350 °C)
a Ammonium desorption calculated from NH3-TPD results.
H-SAPO-34	16 641.1	4144.0	20 785.1
Cu/SAPO-34	15 873.8	1359.0	17 232.8
Cu/ZS-PM-15%	13 688.6	1332.5	15 021.1
Cu/ZS-PM-25%	11 882.3	1330.4	13 212.7
Cu/ZS-PM-35%	12 879.6	1333.2	14 212.8
Cu/ZS-PM-50%	11 802.4	1292.8	13 095.2
Cu/ZS-MM-15%	9245.4	436.7	9682.1

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3.7. UV-vis DRS analysis

UV-vis DRS spectra of pure SAPO-34, Cu/SAPO-34, Cu/ZS-PM-x and Cu/ZS-MM-15% samples have also been performed and the results are displayed in Fig. 7. The bands at 245 and 285 nm are assigned to pure SAPO-34 zeolite. For Cu/SAPO-34, Cu/ZS-PM-x composites and Cu/ZS-MM-15% samples, two main characteristic absorption bands appear. The one absorption band is centered at ∼220 nm and the other one observed in the 600–800 nm region is centered at 750 nm. The former one is assigned to charge transfer lattice O²⁻ → Cu²⁺. The latter one is ascribed to d–d Cu²⁺ transition in dispersed CuO particles.³⁸ Furthermore, no adsorption band is observed at 650 nm with respect to bulk CuO,³⁹ which is in accordance with the results of XRD. Moreover, it's worth noting that a red shift of copper adsorption edge was observed with the increasing of mass fraction of ZSM-5 in Cu/ZS-PM-x catalysts, indicating that a decrease in the dispersion of Cu species resulted from the decrease of specific surface area with increasing the content of ZSM-5 seeds. Additionally, compared with Cu/ZS-PM-x catalysts, Cu/ZS-MM-15% shows a red shift of copper adsorption edge, indicating the worse active components dispersion on Cu/ZS-MM-15% catalyst.

Fig. 7 UV-vis DRS spectra of pure H-SAPO-34, Cu/SAPO-34, Cu/ZS-PM-x and Cu/ZS-MM-15%.

3.8. H₂-TPR results

H₂-TPR curves of Cu/SAPO-34, Cu/ZS-PM-x and Cu/ZS-MM-15% catalysts are shown in Fig. 8. TPR peaks for dehydrated samples are assigned as follows:^40–42 the feature centered at ∼270 °C is attributed to the reduction of dehydrated Cu²⁺ ions in the CHA cage (Cu²⁺ → Cu⁺); the feature at ∼220 °C is assigned to the reduction of dispersed bulk CuO (Cu²⁺ → Cu⁺); the feature at ∼350 °C is due to the reduction in dehydrated Cu²⁺ ions within the D6R (Cu²⁺ → Cu⁺), and the high-temperature feature at around 430 °C corresponds to the reduction of Cu⁺ to Cu⁰. It should be noted that all of the catalysts show similar reduction features. However, notable differences can also be found for feature assigned to the reduction of dehydrated Cu²⁺ ions in the CHA cage, which are considered as the key active sites for the reduction of NO_x. With the addition of ZSM-5, the temperature was shifted from 275 (288) to 266 (280) °C, but further increasing the amount of ZSM-5 seeds, the peak temperature gradually shifted to 278 °C. Meanwhile, the intensity of the low-temperature feature at ∼220 °C sharply decreased and the peak was shifted towards higher temperature as the amount of doped ZSM-5 above 35%. Based on the above results, it is concluded that only appropriate amount of doped ZSM-5 seeds can enhance the redox property of catalysts, which greatly improve its SCR activity leading to higher NO conversion compared with Cu/SAPO-34 catalyst. Compared with Cu/SAPO-34 and Cu/ZS-PM-x catalysts, Cu/ZS-MM-15% displays a much lower temperature with respect to the reduction of dispersed bulk CuO (Cu²⁺ → Cu⁺) and Cu⁺ to Cu⁰. It is noted that the CuO nanoparticles favors the reduction of NO at lower temperature, due to that it could promote the oxidation of NO to NO₂ thus facilitating the “fast SCR” process. However, a lower reduction temperature of Cu⁺ to Cu⁰ is unbeneficial to the stability of NH₃-SCR activity at high temperature.

Fig. 8 H₂-TPR profiles of Cu/SAPO-34, Cu/ZS-PM-x and Cu/ZS-MM-15% catalysts.

3.9. In situ DRIFTS spectra results

3.9.1. Adsorption of NH₃. The surface acid sites and their acidity can be obtained by studying the DRIFTS spectra of the adsorbed NH₃. Fig. 9 shows the DRIFT spectra of Cu/ZS-PM-15% catalysts treated in flowing 1000 ppm NH₃/N₂ for 60 min and then purged in N₂ at different temperatures. As can be seen from Fig. 9, the band at 1460 cm⁻¹ is due to the bending vibration of NH⁴⁺ on the Brønsted acidic sites, while the bands at 1267 and 1617 cm⁻¹ can be assigned to bending vibrations of the N–H bonds in the NH₃ coordinated to the Lewis acidic sites (copper ion sites).^43,44 The bands at 3343 and 3270 cm⁻¹ can be attributed to ammonium ions, and the band at 3180 cm⁻¹ is assigned to coordinated ammonia. The negative peaks at 3677 cm⁻¹ can be assigned to the O–H stretching bands of surface silanol and 3598 cm⁻¹ should be structural hydroxyl groups on Cu/ZS-PM-15% catalyst.⁴⁵ With the increasing of the temperature, the intensities of all the bands decreased, indicating the desorption of ammonia. Moreover, IR bands of NH⁴⁺ on the Brønsted acidic sites (1460 cm⁻¹) disappeared almost completely above 300 °C, while the bands of the coordinated NH₃ (1267 cm⁻¹) can be still detected at 500 °C. The above results suggest that ammonia should be adsorbed on the both Lewis and Brønsted acidic sites for Cu/ZS-PM-15% catalyst. And the ammonia on the Brønsted acid sites was not as stable as that on the Lewis acidic sites, easily desorbing up to 300 °C.

Fig. 9 DRIFTS spectra of chemisorbed 1000 ppm NH₃/N₂ on Cu/ZS-PM-15% catalyst at 100 °C followed by purge in N₂ at different temperatures.

3.9.2. Reaction between NO + O₂ and pre-adsorbed NH₃. The in situ DRIFT spectra of Cu/ZS-PM-15% catalysts as a function of time in flowing of NO + O₂ are shown in Fig. 10. It was noted that both Brønsted (1460 cm⁻¹) and Lewis (1267 and 1617 cm⁻¹) acid sites are detected when the sample was purged by NH₃. After Cu/ZS-PM-15% was exposed to NO and O₂, the intensity of the bands ascribed to adsorbed NH₃ species gradually decreased and even disappeared within 40 min. Meanwhile, some new bands assigned to surface adsorbed NO₂ (1627 cm⁻¹), bidentate nitrate (1575 and 1557 cm⁻¹) and monodentate nitrate (1596 cm⁻¹) appeared.^46–48 With the increasing of reaction time, some of the bands gradually became weak and even vanished. However, the intensity of the band attributed to surface adsorbed NO₂ at 1627 cm⁻¹ was still strong even after 60 min. It further demonstrates that NO₂ species is a primary intermediate following NO adsorption on Cu/ZS-PM, suggesting that it should be directly involved in the “fast” SCR reaction.^49–51 Combined the results of NH₃-SCR performance, it is reasonable to deduce that NO₂ may participate in the “fast step” and promote the reaction rate. Therefore, the NO conversion is higher on Cu/ZS-PM-15% at low temperature.

Fig. 10 In situ DRIFT spectra over Cu/ZS-PM-15% catalyst as a function of time in a flow of NO + O₂ after the catalysts was pre-exposed to a flow of NH₃ for the 60 min and followed by N₂ purging for 30 min at 150 °C.

4. Conclusions

A series of ZSM-5/SAPO-34 composites with different amount of ZSM-5 seeds were controllably synthesized by a pre-seed method. These materials combine the advantage of both ZSM-5 and SAPO-34, which should contribute to reduce diffuse resistance and then improve the accessibility of reactants to catalytically active sites. The redox property of catalysts was significantly influenced by the content of ZSM-5 seeds. However, there is no obvious correlation between acid strength and NH₃-SCR performance. The in situ DRIFTS spectra results revealed that both Lewis and Brønsted acid site are involved in NH₃-SCR reaction and the reaction on Brønsted acid sites appears to be more active than that on Lewis acid sites. NO₂ species is a principal intermediate following NO adsorption on Cu/ZS-PM catalysts, which may facilitate “fast” SCR reaction at low temperature.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21376261 and 21173270), 863 Program of China (2015AA034603), the Beijing Natural Science Foundation (2142027) and the China University of Petroleum Fund (20130007110007 and 2462015QZDX04).

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