SAPO-34 with hierarchical pore structure

Chemical Engineering Journal 379 (2020) 122376

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Excellent selective catalytic reduction of NOx by NH3 over Cu/SAPO-34 with hierarchical pore structure ⁎

Rui Lia,b, Peiqiang Wanga, Shibo Maa, Fulong Yuana, Zhibin Lia, , Yujun Zhua,

T

⁎

a Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education, School of Chemistry and Materials, Heilongjiang University, Harbin 150080, PR China b MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, Environment and Energy Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, PR China

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

of Cu/SAPO-34-H with • Preparation hierarchical pore structure by ion ex-

Cu ion-exchanged SAPO-34 with hierarchical pore structure (Cu/SAPO-34-H) presents excellent NH3-SCR activity, which is attributed to the active Cu species located in D6R.

changed method.

High NH -SCR of NO activity over • 2.7 wt%Cu/SAPO-34-H at 170–480 °C 3

x

even under 10%H2O.

SCR activity due to Cu • Excellent mainly located in D6R in Cu/SAPO-

2+

34-H.

pore structure promote • Hierarchical the generation stability and of active Cu species.

better hydrothermal stability at • Much either high temperature or low temperature.

A R T I C LE I N FO

A B S T R A C T

Keywords: Selective catalytic reduction of NOx Hierarchical pore structure Cu/SAPO-34 Ion exchanged method Good resistance to hydrothermal aging

SAPO-34 with hierarchical pore structure (SAPO-34-H) was first prepared by a hard-template method using CaCO3 as template. The xCu/SAPO-34-H (x = 1.1 wt%, 1.9 wt%, 2.7 wt% and 3.2 wt%) catalysts were synthesized by ion-exchanged method and applied for selective catalytic reduction (SCR) of NOx with NH3. XRD, XRF, SEM, TEM, UV–vis-DRS, NH3-TPD, XPS, NMR, EPR and in situ DRIFTS were used to characterize the physiochemical properties of the catalysts. The SCR activity results demonstrate that 100% NOx conversion can be achieved over the 2.7 wt%Cu/SAPO-34-H catalyst at about 150 °C in the GHSV of 40,000 h−1. Furthermore, NOx conversion under the testing conditions of H2O and SO2 was slightly influenced over the 2.7 wt%Cu/SAPO-34-H catalyst after high-temperature hydrothermal aging at 700 and 800 °C, respectively. NOx convention presents above 90% at 170–480 °C even introducing 10% H2O for 10 h, and it can be maintained 90% by introducing 200 ppm SO2 and 12% H2O over 2.7 wt%Cu/SAPO-34-H. In the meantime, 2.7 wt%Cu/SAPO-34-H was investigated through the low-temperature hydration treatment at 80 °C, it also displays superior hydration treatment resistance for low-temperature. The outstanding performance is ascribed to the hierarchical pore structure of 2.7 wt%Cu/SAPO-34-H, which can promote the diffusion of reaction gas and generation of active sites. EPR results indicate that the Cu2+ species mainly locates the site I in 2.7 wt%Cu/SAPO-34-H, while the Cu2+ species mainly locates the site III for the conversional 2.7 wt%Cu/SAPO-34. Moreover, the in situ DRIFTS results indicate that the Lewis acid sites are easy to participate in NH3-SCR reaction for 2.7 wt%Cu/SAPO-34-H, which mainly follows the Eley-Rideal (E-R) reaction mechanism at 200 °C for 2.7 wt%Cu/SAPO-34-H.

⁎

Corresponding authors. E-mail addresses: [email protected] (Z. Li), [email protected] (Y. Zhu).

https://doi.org/10.1016/j.cej.2019.122376 Received 3 June 2019; Received in revised form 27 July 2019; Accepted 29 July 2019 Available online 30 July 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Chemical Engineering Journal 379 (2020) 122376

R. Li, et al.

1. Introduction

As we know, the BET surface area and mass transfer ability can be enhanced by the formation of hierarchical pore structure in SAPO-34, which is favor of the NH3-SCR activity. In this work, SAPO-34 with hierarchical pore structure (SAPO-34-H) was synthesized by hardtemplate method, and a serial of xCu/SAPO-34-H (x = 1.1 wt%, 1.9 wt %, 2.7 wt% and 3.2 wt%) catalysts with different Cu content were prepared by ion-exchanged method and applied for NH3-SCR of NOx reaction. Among these xCu/SAPO-34-H catalysts, the 2.7 wt%Cu/ SAPO-34-H exhibited outstanding catalytic activity in a wide temperature window, high hydrothermal stability and excellent H2O/SO2 durability.

Nitrogen oxides (NOx) is the main air pollutant from stationary and automobile exhaust, which can give arise to a lot of environmental problems, for instance haze issues, global warming as well as ozone depletion. At present, selective catalytic reduction with ammonia (NH3SCR) is considered to be an effective method to abate NOx pollutant from diesel engine [1–3], which is composed of diesel particular filter (DPF), diesel oxidation catalyst (DOC) and SCR catalyst [4,5]. The exhaust temperature of DPF can reach up to above 700 °C, whereas SCR catalyst is usually placed at the back of DPF, therefore, the excellent hydrothermal durability is an important performance index for SCR catalyst. The commercial NH3-SCR catalysts including V2O5-WO3-TiO2 and V2O5-MO3-TiO2 have the limitation of biotoxicity of V species and the narrow operation temperature windows of NOx conversion (300–400 °C) [6–10]. Hence, it is very necessary to develop an environment-friendly catalyst with excellent activity and N2 selectivity in application of NOx pollutant removal from diesel engine. Recently, the Cu, Mn and Fe modified zeolite catalysts such as Beta (BEA) [11–13], ZSM-5 (MFI) [14–16], SAPO-44 [17–19], SSZ-13 and SAPO-34 (CHA) [20–25] have been widely concerned for removing diesel emissions. However, the Cu/ZSM-5 and Cu/BEA catalysts are prone to the deactivation reaction, which is dealt with high-temperature filter aging. The Cu ion-exchanged SSZ-13 and SAPO-34 catalysts have received particular popularity due to their superior activity even after high temperature treatment. However, the Cu/SSZ-13 was synthesized by using N, N, N-trimethyl ammonium hydroxide ammonium hydroxide as a structure directing agent, the price of which is relatively high and it is not applicable to industrial applications. Xiao’s group prepared Cu-SSZ-13 by using low-cost template of Cu-TEPA, which has demonstrated excellent NH3-SCR performance that over 90% NOx conversion can be achieved in the temperature range of 200–450 °C [26]. Cu/SAPO-34 has been received wide attentions due to the good hydrothermal stability, the microporous structure of SAPO-34 that is beneficial to prevent the production of by-products [27,28]. However, it is a significant issue how to improve the NOx conversion with wide temperature window, the H2O/SO2 resistance especially after lowtemperature hydration treatment for Cu/SAPO-34 catalyst [29,30]. Nowadays, some researchers have devoted to studying Cu/SAPO-34 catalyst. Liu et al studied the influence of low-temperature hydration treatment on the activity of Cu/SAPO-34, which demonstrated that the activity was influenced greatly by introducing H2O at 80 °C [31]. Wang et al. prepared a series of Cu/SAPO-34 with the different Si contents, in which Cu1.264/Si0.374/Al0.441 showed the best catalytic activity in the whole temperature range [32]. However, the NOx conversion can only achieve 90% at 250 °C. Wang and co-work utilized in situ-DRIFTS to analysis the reaction mechanism of Cu/SAPO-34 catalyst. The in situ DRIFTS results revealed that both Lewis and BrØnsted acid sites played important roles in the NH3-SCR reaction, NOx can readily reacted with surface NH3 to form NH4NO2 and NH4NO3 on the Lewis acid sites, and the BrØnsted acid sites can reserve a large amount of NH3 [33]. In past years, SAPO-34 with hierarchical pore structure has been concerned for NH3-SCR reaction. Liu et al. synthesized a series of mesomicroporous Cu/SAPO-34 by a one-step hydrothermal method, in which the 1.0-Cu/SAPO-34 presented the excellent catalytic activity (> 90% NO conversion) in the temperature range of 150–425 °C, however, the SO2 and H2O resistance as well as the hydrothermal stability were not mentioned [34]. Zhang et al. prepared core-shell structured Cu/SSZ-13 by using the NaOH solution, where the core-shell structured Cu/SSZ-13 exhibited high hydrothermal stability and low temperature activity. However, the H2O and SO2 resistance were also not discussed in this work [35].

2. Experimental 2.1. Preparation of SAPO-34 with hierarchical pore SAPO-34 zeolite with hierarchical pore was prepared by hydrothermal synthesis method with hard template treatment. First, 3.5 g pseudoboehmite (71 wt% Al2O3, Kermel company) was dissolved in 30 mL distilled water and stirred for 2 h at room temperature. Second, 3.3 mL H3PO4 (85 wt%, Kermel company) was added to the solution and stirred 3 h. Third, 1.5 g of SiO2 was added to the mixture until the power was dissolved under stirring. 3.8 mL morpholine were used as the structure-directing agent (SDA), which was introduced into the solution and stirred overnight. Finally, 2 g of CaCO3 was added to the solution and stirred one day. The resulting gel was transferred to a stainless autoclave with 50 mL and heated at a temperature of 200 °C for 48 h. The obtained crystalline products were centrifuged and washed with HCl solution (0.25 mol/L) to remove CaCO3, and then washed with distilled water for three times. The product was dried at 110 °C overnight and calcined at 550 °C for 5 h to get SAPO-34 with hierarchical pore, which was donated as SAPO-34-H. For comparison, the traditional SAPO-34 was synthesized according to the similar ratio of Si/Al/P without adding CaCO3. 2.2. Preparation of the catalysts Cu/SAPO-34-H with hierarchical pore series catalysts were prepared by ion-exchanged method using different copper content. The obtained hierarchical pore SAPO-34-H was firstly exchanged with 0.1 mol/L NH4NO3 aqueous for 1 h at 70 °C twice, then centrifuged and dried at 110 °C overnight. The obtained NH4+-SAPO-34-H was exchanged with different loading Cu at 70 °C for 12 h, and the product was calcined at 550 °C for 5 h. The obtained catalysts were denoted as xCu/SAPO-34-H (x = 1.1 wt%, 1.9 wt%, 2.7 wt% and 3.2 wt%), in which x presented the mass fraction of copper. For comparison, the ordinary Cu/SAPO-34 catalyst was also prepared in the basis of the above similar route. 2.3. Catalysts characterization X-ray diffraction (XRD) measurement in the 2θ range of 5–40° was performed on Bruker D8 Advance X-ray diffract meter with Cu Ka radiation (λ = 0.1542 nm) and the scanning speed of 0.02°·S−1. Scanning electron microscope (SEM) images were tested using a Hitachi S-4800 microscope at 20 kV and transmission electron microscope (TEM) images were carried out on JEM-2010 (JEOL) equipment with the worked voltage of 200 KV. X-ray fluorescence (XRF) spectrometer was used to measure the loading amount of Cu and elemental composition of zeolite by Bruker S4 Explorer instrument. BET surface area (SBET) and pore characterization were analyzed by N2 adsorption-desorption at −196 °C with 2

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follows: center field 3500 G, sweep width 3000 G, static field 2000 G as well as the frequency of Microwave Bridge was 9.76 GHz. In situ DRIFTS was tested on FTIR spectrometer (Nicolet 6700), which equipped with high-sensitive MCT detector cooled by liquid N2. Firstly, the catalyst was loaded on the IR cell (Harrick) and the temperature was increased to 400 °C and kept for 1 h in the N2 atmosphere in order to remove adsorbed gas on catalyst, and then the temperature was decreased to 130 °C to collect the background. The in situ DRIFTS need to subtract the background spectra, which is recorded by collecting 64 co-added scans with 4 cm−1 resolution. The catalyst was exposed to the gas conditions of 1000 ppm NO, 1000 ppm NH3, 3% O2 and N2 as balance.

Quantachrome Aotosorb iO-MP, which can give the corresponding micropore data including surface area, pore volume and pore size distribution. Before measurement, the sample (0.100 g) was treated with heating at 200 °C for 5 h in the vacuum condition in order to remove contaminants and impurities. 29 Si, 27Al and 31P nuclear magnetic resonance (NMR) were performed on Bruker Avance III 400 spectrometer. 29Si chemical shift was referred to Kaolin, 27Al chemical shift was referred to Al(NO3)3 of 1 mol/L, and 27Al NMR data were recorded with a spinning rate of 12 kHz. 31P chemical shift was consulted to H3PO4 (85 wt%), 29Si NMR and 31P NMR spectra were recorded with a spinning rate of 6 kHz. Temperature programmed desorption of ammonia (NH3-TPD) measurements were performed on AutoChem TP5080 instrument. Firstly, the obtained product (0.100 g) was added to a sample tube, in which the sample was pretreated in a flow of He gas at 400 °C for 1 h. Secondly, the temperature was decreased to 100 °C in the He gas atmosphere. Then the sample was adsorbed with pure NH3 (25 mL/min) for 30 min. Finally, the temperature was elevated to 600 °C at a rate of 10 °C/min in the flow (30 mL/min) of He gas. UV–visible diffuse reflectance spectra (UV–vis-DRS) were carried out on Perkin-Elmer Lambda950, in which BaSO4 was used as reference. X-ray photoelectron spectroscopy (XPS) experiments were taken on the spectrum (Kratos-AXIS ULTRA DLD) with an Al Kα radiation source. The binding energy of C1s at 284.4 eV was used as reference. The electron paramagnetic resonance (EPR) tests were carried out on a Bruker EPR A200 instrument. The test condition was as

2.4. Catalytic performance measurement NH3-SCR activity was carried out on a fixed-bed quartz tubular flow reactor (i.d. 5 mm), the catalyst was added into the reactor. The inlet mixture gases were composed of 1000 ppm NH3, 1000 ppm NO, 3 vol% O2 and N2 as balance gas. The gas flow was dominated at 200 mL/min in the GHSV of 40,000 h−1 as well as 600 mL/min in the GHSV of 240,000 h−1, respectively. The content of the outlet gas (NH3, NO, N2O and NO2) were measured using standard curve by FTIR spectrometer (Nicolet 6700) equipped with a gas cell (PIKE). The NOx conversion and N2 selectivity were calculated by below Eqs. (1) and (2):

Fig. 1. NOx conversion over 2.7wt%Cu/SAPO-34 and xCu/SAPO-34-H in the GHSV of 40,000 h−1 (A), Effects of GHSV (B), Hydrothermal aging (C) and SO2, H2O (D) on NOx conversion over 2.7 wt%Cu/SAPO-34-H. 3

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temperature, and the temperature above 90% NOx conversion is in range of 200–500 °C. The above results demonstrate that 2.7 wt%Cu/ SAPO-34-H exhibits great catalytic activity even under the much higher GHSV of 240,000 h−1.

NOx conversion = 100% × (1 − ([NO]out + [NO2 ]out )/([NO]in + [NO2 ]in ))

(1)

N2 selectivity (%) = 100% × ([NO]in − [NO]out + [NH3 ]in − [NH3 ]out − [NO2 ]out − 2[N2 O]out ) / ([NO]in − [NO]out + [NH3 ]in − [NH3 ]out )

3.1.3. Effect of hydrothermal aging over 2.7 wt%Cu/SAPO-34-H In order to study the hydrothermal ability of catalyst, the SCR activity over the 2.7 wt%Cu/SAPO-34-H catalyst after hydrothermal aging at high temperature was measured under the GHSV of 40,000 h−1, and the results are revealed in Fig. 1(C). The catalyst is treated in N2 condition with 10% H2O at 700 and 800 °C for 12 h, respectively. The NOx conversion displays slightly decrease at low temperature after the sample is treated with 10% H2O at 700 °C for 12 h, which can still achieve 90% from 170 °C to 500 °C. Then the aging temperature is further increased to 800 °C, it can clearly be seen that the NOx conversion slightly decreases in the temperature range of 100–200 °C, meanwhile the above 90% NOx conversion is still maintained in the temperature range of 170–480 °C. The catalytic performance of the 2.7 wt%Cu/SAPO-34 and 2.7 wt%Cu/SAPO-34-H catalysts under as high as 120,000 h−1 of GHSV was also further studied after hydrothermal treatment at 750 °C presented in Fig. S3. It can be observed that NOx conversion (> 95%) over 2.7 wt%Cu/SAPO-34 is at the temperature window of 200–350 °C under the GHSV of 120,000 h−1, and the NOx conversion reduces below 200 °C when 2.7 wt%Cu/SAPO-34 is treated with 10% H2O at 750 °C. While the 2.7 wt%Cu/SAPO-34-H catalyst exhibits good activity window after hydrothermal treating by 750 °C, in which the NOx conversion can get to above 90% in the temperature range of 250–500 °C. The above results suggest that the 2.7 wt%Cu/SAPO-34-H catalyst has excellent hydrothermal aging resistance even though the GHSV of 120,000 h−1. In addition, it was reported that low-temperature hydration treatment has a severe impact on NH3-SCR activity of SAPO-34 [31]. Fig. S4 presents the NOx conversion over 2.7 wt%Cu/SAPO-34 and 2.7 wt %Cu/SAPO-34-H after hydration treatment at 80 °C for 2 h. It can be observed that the NOx conversion for 2.7 wt%Cu/SAPO-34-H shows a slightly influence at low temperature below 200 °C, and the 100% NOx conversion can still be achieved in the temperature range of 200–500 °C. However, comparing with 2.7 wt%Cu/SAPO-34-H, the low temperature hydration treatment has a severe influence on NOx conversion for 2.7 wt%Cu/SAPO-34, in which the NOx conversion decreases from 98% to 69%. The results indicate that the 2.7 wt%Cu/ SAPO-34-H catalyst exhibits the satisfactory low-temperature hydration resistance.

(2)

where [NO2]in, [NH3]in and [NO]in in above formulas refer the concentrations of inlet gas for NH3 and NO, respectively; [NO]out, [NH3]out [N2O]out, [NO2]out and [NO]out in above equation present to concentrations of outlet gas for NO, NH3, N2O, NO2 and NO, respectively. 3. Results and discussion The SAPO-34 with hierarchical pore structure (SAPO-34-H) was first synthesized by hard-template method. The conventional SAPO-34 as support was also prepared. The xCu/SAPO-34-H (x = 1.1 wt%, 1.9 wt %, 2.7 wt% and 3.2 wt%) and 2.7 wt%Cu/SAPO-34 catalysts were prepared by ion-exchanged method, and the NH3-SCR activities of xCu/ SAPO-34-H and 2.7 wt%Cu/SAPO-34 were compared. 3.1. NH3-SCR activity 3.1.1. NOx conversion over xCu/SAPO-34-H Fig. 1(A) shows NOx conversion over the 2.7 wt%Cu/SAPO-34 and xCu/SAPO-34-H (x = 1.1 wt%, 1.9 wt%, 2.7 wt% and 3.2 wt%) catalysts with the temperature change. For 1.1 wt%Cu/SAPO-34-H, it can be clearly seen that the NOx conversion increases with the reaction temperature from 100 °C. When the temperature gets 250 °C, the NOx conversion can achieve 98% that can be maintained in the temperature range of 250–500 °C. For different Cu content in xCu/SAPO-34-H, it is noticed that NOx conversion increases at low temperature with increasing the Cu content, moreover, the temperature near 100% NOx conversion decreases with the increase in the Cu amount. Meanwhile, 2.7 wt%Cu/SAPO-34-H, 2.7 wt% of ion-exchanged Cu amount, exhibits the excellent NH3-SCR activity that above 90% NOx conversion can be achieved from 150 °C to 480 °C. When the Cu amount is 3.2 wt%, the NOx conversion decreases at low temperature, which gets more over 90% in the temperature range of 160–480 °C. In order to understand the influence of the hierarchical pore structure on the NH3-SCR reaction, the NOx conversion over 2.7 wt%Cu/SAPO-34 was measured for comparison. It can be observed that 100% NOx conversion is obtained at about 200 °C over 2.7 wt%Cu/SAPO-34, and above 90% NOx conversion is in the temperature range of 200–350 °C. These results indicate that Cu exchanged SAPO-34-H zeolite with hierarchical pore structure can obtain excellent SCR performance. N2 selectivity is a significant impact to evaluate NH3-SCR catalyst. As shown in Fig. S1, the N2 selectivity gradually decreases with increasing temperature, and it is only about 70% at 500 °C over 2.7 wt%Cu/SAPO-34. While the N2 selectivity presents above 95% over the Cu/SAPO-34-H series catalysts. The activity stability of 2.7 wt%Cu/SAPO-34-H is presented in Fig. S2. It can be observed that the NOx conversion is unchanged in whole testing temperature range from 100 °C to 500 °C over the 2.7 wt%Cu/SAPO-34H catalyst after a month’s storage in the air compared with the fresh 2.7 wt%Cu/SAPO-34-H catalyst.

3.1.4. Effect of SO2 and H2O over 2.7 wt%Cu/SAPO-34-H The water vapor and sulfur dioxide exist normally in exhaust fumes, and they can influence the NH3-SCR activity. The ability of SO2 and H2O tolerance is a main factor to evaluate a catalyst in the NH3-SCR reaction. Fig. 1(D) shows the SO2 and H2O resistance over the 2.7 wt %Cu/SAPO-34-H catalyst with different concentrations of SO2 and H2O at 200 °C under the GHSV of 40,000 h−1. Firstly, 6% H2O is added to the reaction gas for about 10 h, it is noticed that there is no influence on the NOx conversion. Then, 100 ppm SO2 is added to the reaction feed for 10 h, the NOx conversion is still maintained to 100%. When the amount of H2O is increased to 12%, the NOx conversion is also unchanged. However, the NOx conversion decreases to about 90% with the addition of 200 ppm SO2 and 12% H2O. After turning off SO2 and H2O, the NOx conversion recovers 100% speedily. In addition, the SO2 and H2O tolerance under 120,000 h−1 of GHSV is also carried out over the 2.7 wt%Cu/SAPO-34-H catalyst at 200 °C. As shown in Fig. S5, the 2.7 wt%Cu/SAPO-34-H catalyst displays the much stronger resistance of SO2 and H2O even though the amount of 100 ppm SO2 and 12% H2O is introduced to the reaction gas. When 12% H2O is added to reaction gas, the NOx conversion maintains 100% about 22 h, then 100 ppm SO2 is introduced to the reaction gas, the NOx conversion decreases to 84% and keeps for 28 h. However, 12% H2O and 100 ppm SO2 are removed

3.1.2. Effect of GHSV over 2.7 wt%Cu/SAPO-34-H The NOx conversion under different GHSV is presented in Fig. 1(B) for the 2.7 wt%Cu/SAPO-34-H catalyst. It can be noticed that the increase of GHSV can lead to the decrease of NOx conversion at low temperature NOx (< 200 °C), meanwhile, the NOx conversion shows slightly increase at high temperature (> 450 °C). NOx conversion (> 90%) can be maintained at a wide temperature window from 180 °C to 480 °C at the GHSV of 80,000 h−1. When the GHSV is increased to 120,000 and 240,000 h−1, the NOx conversion slightly decreases at low 4

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changed in their peak intensity and peak position, which moves towards the low angle compared with SAPO-34-H. The above results demonstrate that peak position moves due to the generation of hierarchical pore and ion-exchanging with Cu. Meanwhile, no diffraction peaks of CaCO3 at 2θ of 23.1, 29.4 and 36.0° (PDF 24-0027) and CuO at 2θ of 35.6 and 38.8° (PDF 48-1548) [37,38] species are observed, which suggests that the CaCO3 species has been completely removed out and Cu species may be dispersed well.

from the reaction feed, the NOx conversion recovers near 100%. The above results demonstrate that 2.7 wt%Cu/SAPO-34-H has much higher SO2 and H2O resistance and excellent reproducible ability. 3.2. NH3 oxidation over xCu/SAPO-34-H In order to study the oxidation ability of the xCu/SAPO-34-H catalysts, the NH3 oxidation tests were carried out and presented in Fig. S6. The NH3 conversion increases with elevating temperature for the five catalysts. It can be clearly seen that the NH3 conversion increases with the copper content over 1.1 wt%Cu/SAPO-34-H, 1.9 wt%Cu/ SAPO-34-H and 2.7 wt%Cu/SAPO-34-H at each temperature. Interestingly, the NH3 conversion over 2.7 wt%Cu/SAPO-34-H is similar to that over 3.2 wt% Cu/SAPO-34-H, which begins to convert at 200 °C and gets to 100% at above 480 °C. However, the NH3 conversion for 3.2 wt %Cu/SAPO-34-H is slightly higher than that for 2.7 wt%Cu/SAPO-34-H in the temperature range of 250–450 °C, which can explain that the decrease in NOx conversion at 400 °C for 3.2 wt%Cu/SAPO-34-H due to the NH3 over-oxidation. Meanwhile, the NH3 oxidation ability over 2.7 wt%Cu/SAPO-34 catalyst is also measured for comparison. It can be observed that the NH3 conversion is about above 96% from 250 °C to 500 °C. Combined with the activity results (Fig. 1), it can be concluded that the narrow activity temperature window is derived from NH3 overoxidation for 2.7 wt%Cu/SAPO-34. The above results indicate that the NH3 oxidation ability increases with the amount of Cu for the xCu/ SAPO-34-H catalysts, and the NH3 over-oxidation is not beneficial to the NH3-SCR reaction. In order to investigate the excellent SCR activity of 2.7 wt%Cu/ SAPO-34-H, a serial of characterizations for 2.7 wt%Cu/SAPO-34 and 2.7 wt%Cu/SAPO-34-H was carried out including XRD, N2 adsorptiondesorption, SEM, TEM, NMR, XPS, EPR and CO adsorption of in situ DRIFTS.

3.4. SEM and TEM results The SEM images of the 2.7 wt%Cu/SAPO-34-H, 2.7 wt%Cu/SAPO34, SAPO-34 and SAPO-34-H samples are displayed in Fig. 3. It can be observed that all samples show cubic morphology of characteristic CHA structure. Among these samples, SAPO-34-H exhibits cubic morphology with a uniform size of about 12.7 μm (Fig. 4a and b), the pore structure is observed on the surface of SAPO-34-H. Compared with SAPO-34-H, the surface of SAPO-34 is smooth, and the size of SAPO-34 is about 9.4 μm (Fig. 4c and d). When 2.7 wt%Cu is exchanged, 2.7 wt%Cu/ SAPO-34-H and 2.7 wt%Cu/SAPO-34 have similar the surface to the parents of SAPO-34-H and SAPO-34, respectively, suggesting their morphologies are almost unchanged after exchanged Cu. The TEM images and pore size distribution of 2.7 wt%Cu/SAPO-34H catalyst are displayed in Fig. 4. Some pores are observed and marked in Fig. 5 (b, c and d), it can be observed that the microporous and mesoporous are exhibited in 2.7 wt%Cu/SAPO-34-H with different size range (Fig. 5a–e). The average pore size of 2.7 wt%Cu/SAPO-34-H is approximate 7.3 nm. Meanwhile, the CuO species is not observed, which indicates that the Cu species may be dispersed well. 3.5. Textual properties and compositions The N2 adsorption–desorption isotherms and pore size distribution of 2.7 wt%Cu/SAPO-34 and 2.7 wt%Cu/SAPO-34-H catalysts are depicted in Fig. 5. From Fig. 5(A), it can be observed that the 2.7 wt%Cu/ SAPO-34 catalyst shows the typical type I adsorption–desorption isotherms, whereas the 2.7 wt%Cu/SAPO-34-H reveals the type IV adsorption–desorption isotherms with a hysteresis loop in the range of 0.4 < P/P0 < 0.9. The pore size distribution of 2.7 wt%Cu/SAPO-34 and 2.7 wt%Cu/SAPO-34-H is presented in Fig. 5(B). The uniform mesopores sizes of 2.7 wt%Cu/SAPO-34-H is about ~ 5.5 nm, while 2.7 wt%Cu/SAPO-34 presents typical microporous structure. The above results indicate that the hierarchical pores structure exists in the 2.7 wt %Cu/SAPO-34-H catalyst. The textual properties and chemical compositions of SAPO-34, 2.7 wt%Cu/SAPO-34, SAPO-34-H and 2.7 wt%Cu/SAPO-34-H are

3.3. XRD results The XRD patterns of the 2.7 wt%Cu/SAPO-34-H, 2.7 wt%Cu/SAPO34, SAPO-34 and SAPO-34-H catalysts are presented in Fig. 2. The SAPO-34 displays the representative diffraction peaks of CHA structure at 2θ of 9.6, 12.8, 15.9 and 20.6° (PDF 34–0137) [36]. It can be observed that 2.7 wt%Cu/SAPO-34 compared with SAPO-34 has significant decreased in the crystallinity intensity as well as peak position has obviously moved towards the low angle. While SAPO-34-H possessed hierarchical pore structure also shows characteristic peaks of CHA structure. Comparing with SAPO-34, it could be observed that the SAPO-34-H has moved towards the low angle about 0.05°. For 2.7 wt %Cu/SAPO-34-H, XRD pattern of 2.7 wt%Cu/SAPO-34-H exhibits little

Fig. 2. XRD patterns of SAPO-34-H (a), SAPO-34 (b), 2.7 wt%Cu/SAPO-34-H (c) and 2.7 wt%Cu/SAPO-34 (d). 5

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Fig. 3. SEM images of SAPO-34-H (a, b), SAPO-34 (c, d), 2.7 wt%Cu/SAPO-34-H (e, f) and 2.7 wt%Cu/SAPO-34 (g, h).

Fig. 4. TEM images (a–e) and pore size distribution (f) of 2.7 wt% Cu/SAPO-34-H.

generation of hierarchical pore could make the increase in BET surface area.

illustrated in Table 1. It is observed that the value of Si/Al ratio is 0.19 and 0.18 for SAPO-34 and 2.7 wt%Cu-SAPO-34. While the Si/Al ratio for SAPO-34-H and 2.7 wt%Cu/SAPO-34-H is 0.36 and 0.43, respectively. This indicates that the generation of hierarchical pore structure changes the Si and Al contents. The total BET surface area is 460, 507, 417 and 471 m2/g for SAPO-34, SAPO-H, 2.7 wt%Cu/SAPO-34 and 2.7 wt%Cu/SAPO-34-H, respectively, meanwhile the corresponding microporous BET surface area is 447, 417, 389 and 384 m2/g. It is obvious that the microporous surface area of 2.7 wt%Cu/SAPO-34-H is similar with 2.7 wt%Cu/SAPO-34, however, the generation of mesoporous increases the total BET surface area of 2.7 wt%Cu/SAPO-34-H. The BET surface area of Cu ion-exchanged catalyst slightly decreases compared with parent. The above results can be concluded that the

3.6. NMR results Solid-state NMR measurements were used to understand the differences in the framework of 2.7 wt%Cu/SAPO-34-H and 2.7 wt%Cu/ SAPO-34. As shown in Fig. 6(A), the signal of the 31P NMR spectra at −28.5 ppm is assigned to P(4Al) for both 2.7 wt%Cu/SAPO-34-H and 2.7 wt%Cu/SAPO-34 [38–40]. As presented in Fig. 6(B), for the 27Al NMR spectra, the signal at 30–37 ppm is ascribed tetrahedrally coordinated Al atoms, and the signal at −10 ppm is assigned to the octahedrally coordinated Al atoms [20,41–45]. Compared with 2.7 wt 6

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Fig. 5. N2 adsorption-desorption isotherms (A) and pore size distribution (B) of 2.7 wt%Cu/SAPO-34 (a) and 2.7 wt%Cu/SAPO-34-H (b). Table 1 physiochemical properties of 2.7 wt%Cu/SAPO-34 and 2.7 wt%Cu/SAPO-34-H. Catalysts

SAPO-34 SAPO-34-H 2.7 wt%Cu/SAPO-34 2.7 wt%Cu/SAPO-34-H a b c d e

SBETb (m2/g)

Concentration content (at.%)a SiO2

Al2O3

P2O5

Si/Al

0.17 0.29 0.17 0.32

0.45 0.40 0.47 0.37

0.38 0.31 0.36 0.28

0.19 0.36 0.18 0.43

460 507 417 471

Smicc (m2/g)

447 401 398 393

Total acid amountd (μmol/g)

– 275 284

Copper species (%)e Cu+

Cu2+

– 22.1 56.7

– 77.9 43.3

Component content was determined by the XRF analysis. BET surface area was derived by applying the multi-point BET-model. Microporous surface area was derived from the t-plot model. Data calculated from NH3-TPD curves. Data calculated from XPS.

%Cu/SAPO-34, the 27Al NMR signal of 2.7 wt%Cu/SAPO-34-H exhibits a little difference, suggesting the environment of coordinated Al is changed. As displayed in Fig. 6(C), the 29Si NMR spectra are fitted by deconvolution shown in Fig. S7. The Si signals at −91, −95, −100, −105 and −110 ppm are assigned to Si4Al, Si3Al1Si, Si2Al2Si, Si1Al3Si and Si4Si species [46,47], respectively, and the signals at −80 and −88 ppm are assigned to the structure defects for 2.7 wt%Cu/ SAPO-34-H and 2.7 wt%Cu/SAPO-34. Detailed integration results of 29 Si species are illustrated in Table S1. It can be obviously seen that the 2.7 wt%Cu/SAPO-34-H catalyst with hierarchical pore structure shows the more Si islands than the 2.7 wt% Cu/SAPO-34 catalyst, which should be owe to the increase of Si content and the decrease of Al

content in the formation of hierarchical pore SAPO-34 combined with the 27Al NMR spectra. This result is also confirmed by the elemental composition derived from the XRF spectra (Table 1). 3.7. UV–vis-DRS analysis UV–vis-DRS were carried out to identify different copper species over the 2.7 wt%Cu/SAPO-34 and 2.7 wt%Cu/SAPO-34-H catalysts. As presented in Fig. S8, the above catalysts show three absorption peaks. A strong sharp peak at 218 nm is assigned to the isolated Cu2+ and Cu+ species. And a weaker and broader peak at 274 nm is attributed to the O-Cu-O and Cu-O-Cu species. The peak at 693 nm is ascribed to the CuO

Fig. 6. Solid-state NMR spectra of 2.7 wt%Cu/SAPO-34-H (a) and 2.7 wt%Cu/SAPO-34 (b) ((A) 7

31

P MAS NMR (B)

27

Al MAS NMR (C)

29

Si MAS NMR).

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species [48–52]. It can be observed that the isolated Cu2+ and Cu+ species mainly exist in the 2.7 wt%Cu/SAPO-34 and 2.7 wt%Cu/SAPO34-H catalysts.

2.7 wt%Cu/SAPO-34 catalyst. Combing with the XPS results, it can be concluded that much more Cu+ species exists in 2.7 wt%Cu/SAPO-34H and there are far more amounts of the Cu2+ species in 2.7 wt%Cu/ SAPO-34. The moderate contents of the Cu+ and Cu2+ species in 2.7 wt % Cu/SAPO-34-H catalyst are beneficial to the catalytic cycle of NH3SCR of NOx.

3.8. NH3-TPD analysis The acidity of catalyst plays a significant role in NH3-SCR of NOx reaction. 2.7 wt%Cu/SAPO-34 and 2.7 wt%Cu/SAPO-34-H were evaluated by NH3-TPD tests between 100 and 600 °C. As presented in Fig. S9, the two catalysts demonstrate two desorption peaks in the whole temperature range. According to the literatures [32,53], the low temperature desorption peak at about 170–190 °C is ascribed to the adsorbed NH3 on physical and weak acid sites. While the high temperature desorption peak at 410–430 °C is assigned to the adsorbed NH3 on the strong acid sites [54]. The acid amounts of the two catalysts are revealed in Table 1, which are 284 and 275 μmol/g for 2.7 wt%Cu/ SAPO-34-H and 2.7 wt%Cu/SAPO-34, respectively, suggesting the similar acid amount for them.

3.11.2. NH3 adsorption In situ DRIFTS of NH3 adsorption are displayed in Fig. 9 over the 2.7 wt%Cu/SAPO-34 and 2.7 wt%Cu/SAPO-34-H catalysts at different time. As shown in Fig. 9, several adsorption peaks at 1503 and 1190 cm−1 are observed that are ascribed to the NH3 adsorption on Lewis acid sites (1190 cm−1) as well as the NH3 adsorption on Brønsted acid sites (1503 cm−1) [11,67–70], revealing the existence of both Lewis and Brønsted acid sites on surface of 2.7 wt%Cu/SAPO-34 and 2.7 wt%Cu/SAPO-34-H. The total acid amount is similar for 2.7 wt %Cu/SAPO-34 and 2.7 wt%Cu/SAPO-34-H from NH3-TPD results. But the ratio of Lewis acid sites to Brønsted acid sites is calculated to 3.66 and 7.13 from the NH3 adsorption curves (Fig. 9) for 2.7 wt%Cu/SAPO34 and 2.7 wt%Cu/SAPO-34-H, respectively. It can be observed that the amount of Brønsted acid sites for 2.7 wt%Cu/SAPO-34 is more than that for 2.7 wt%Cu/SAPO-34-H, which is due to the increase of Si and the decrease of Al in the formation of SAPO-34-H.

3.9. XPS analysis The chemical state of the Cu species was analyzed by XPS for 2.7 wt %Cu/SAPO-34 and 2.7 wt%Cu/SAPO-34-H, and the results are shown in Fig. S10. Meanwhile, atomic concentration of surface composition was calculated and displayed in Table 1. For 2.7 wt%Cu/SAPO-34, the Cu 2p spectrum is fitted into four peaks by deconvolution, which are assigned to Cu+ (932.4 and 953.0 eV) and Cu2+ (934.1 and 954.2 eV) [55–61], and the amount of Cu+ and Cu2+ are calculated to 22.1% and 77.9%, respectively. For the 2.7 wt%Cu/SAPO-34-H catalyst, the Cu 2p spectrum is also deconvoluted into four peaks at 932.4, 952.3, 934.3 and 955.1 eV, which are ascribed to the Cu+ and Cu2+ species too, and the responding amount is 56.7% and 43.3%, respectively. The above results demonstrate that the 2.7 wt%Cu/SAPO-34 catalyst shows more surface Cu2+ species compared with 2.7 wt%Cu/SAPO-34-H, and the 2.7 wt%Cu/SAPO-34-H possesses suitable amount of Cu+ and Cu2+, which was beneficial to the oxidation and reduction cycle reaction.

3.11.3. Reaction between NOx and adsorbed ammonia In situ DRIFTS of reaction between NOx and pre-adsorbed NH3 on 2.7 wt%Cu/SAPO-34 at 130 °C is depicted in Fig. 10 (A). After adsorption of NH3, several peaks ascribed to the adsorbed NH3 on Lewis acid sites (1190 cm−1) and Brønsted acid sites (1503 cm−1) are detected [11,60,61,67–70]. It can be observed that the peak at 1190 cm−1 disappears after NO + O2 is introduced to the gas cell for about 15 min. However, the peak at 1503 cm−1 disappears after introducing NO + O2 for 12 min. The corresponding in situ DRIFTS over 2.7 wt%Cu/SAPO34-H are depicted in Fig. 10 (B). When the 2.7 wt%Cu/SAPO-34-H is exposed to NH3 for 30 min, the peaks of NH3 adsorbed on Lewis acid sites (1190 cm−1) and Brønsted acid sites (1503 cm−1) are observed. Then, by introducing NO + O2 after 10 min, the peaks of NH3 adsorbed on both Lewis acid sites and Brønsted acid sites disappear. However, the nitrate species is not detected. It can be observed that the NOx species can more easily react with adsorbed on both Lewis acid species and Brønsted acid sites for 2.7 wt%Cu/SAPO-34-H. Furthermore, the reaction rate of acid site with increasing time is calculated and presented in Fig. 11. It can be observed that the NH3 adsorbed on Lewis acid reacts

3.10. EPR analysis EPR tests were performed to analysis the coordination environment of Cu2+ in 2.7 wt%Cu/SAPO-34-H and 2.7 wt%Cu/SAPO-34. As depicted in Fig. 7, the Cu2+ species is in site I (g = 2.386) for 2.7 wt%Cu/ SAPO-34-H, suggesting that the Cu2+ exists on double six-memberedring (D6R) [62]. The 2.7 wt%Cu/SAPO-34 catalyst show the characteristic peak in site III (g = 2.361), inferring that the Cu2+ species is in the CHA cavity along the three-fold [63,64]. The above results indicate that the Cu2+ species exists in different framework location of different SAPO-34 (2.7 wt%Cu/SAPO-34-H and 2.7 wt%Cu/SAPO-34). 3.11. In situ DRIFTS tests In order to understand the effect of hierarchical pore structure on adsorption species and reaction mechanism, in situ DRIFTS tests were performed for the 2.7 wt%Cu/SAPO-34 and 2.7 wt%Cu/SAPO-34-H catalysts. 3.11.1. CO adsorption CO can be used as a probe molecular in DRIFTS to detect the valence of copper species. CO adsorption on 2.7 wt%Cu/SAPO-34 and 2.7 wt %Cu/SAPO-34-H is revealed in Fig. 8. It can be observed that the characteristic peaks at 2159 cm−1 as well as 2138 cm−1 are detected on 2.7 wt%Cu/SAPO-34-H with hierarchical pore, which are ascribed to CO stretching frequency of the Cu+-CO species [65]. And the peaks at 2174 and 2111 cm−1 are assigned to the stretching vibrations of gaseous CO [66]. Compared with 2.7 wt%Cu/SAPO-34-H, it is found that 2.7 wt%Cu/SAPO-34 has only small amount of the Cu+ species in the

Fig. 7. EPR spectra of 2.7 wt%Cu/SAPO-34-H (a) and 2.7 wt%Cu/SAPO-34 (b) catalyst. 8

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(700 and 800 °C) as well as SO2/H2O resistance. It is worth noting that the above good performance can be obtained under a relatively high GHSV. However, 2.7 wt%Cu/SAPO-34 shows only the above 90%NOx conversion in the temperature range of 200–350 °C. The catalytic performance of 2.7 wt%Cu/SAPO-34-H is also compared with that of other Cu modified zeolite catalysts displayed in Table S2. Interestingly, the 2.7 wt%Cu/SAPO-34-H displays excellent catalytic activity in the similar conditions. Additionally, Wijayanti et al. [71] have also studied the influence of sulfur oxide on NOx conversion over Cu/SAPO-34, showing the deactivation phenomenon with the addition sulfur oxide in the feed. However, the sulfur oxide has no impact on the activity over 2.7 wt%Cu/SAPO-34-H when introducing 100 ppm SO2, 100% NOx conversion is still maintained at 200 °C. The above results demonstrate that 2.7 wt%Cu/SAPO-34-H is a potentially practical catalyst for NH3SCR of NOx. Based on the comparisons of activity and characterization between 2.7 wt%Cu/SAPO-34-H and 2.7 wt%Cu/SAPO-34, it can be concluded that the excellent NH3-SCR performance of 2.7 wt%Cu/ SAPO-34-H is related to the special hierarchical pore structure. The results of XRD (Fig. 2), N2 adsorption–desorption isotherms (Fig. 5), SEM (Fig. 3) and TEM (Fig. 4) confirm that the 2.7 wt%Cu/ SAPO-34-H processes hierarchical pore structure with regular cubic morphology. The XRF results testify that the formation of SAPO-34-H with hierarchical pore structure leads to the changes of Si and Al contents, generating more Si island compared with 2.7 wt%Cu/SAPO-34, which is verified by NMR results (Fig. 6). The changes of Si and Al contents lead to change the coordination environment of Si and Al in SAPO-34-H, which can enhance the hydrothermal aging resistance whatever the treatment condition is at high temperature or low temperature for xCu/SAPO-34-H. Furthermore, the change of microstructure for the SAPO-34-H support results in the different exchanged copper species that is the key factor in the special NH3-SCR activity for 2.7 wt%Cu/SAPO-34-H. The XPS results reveal a large number of Cu2+ on the surface of the 2.7 wt %Cu/SAPO-34 catalyst (Table 1 and Fig. S10) which presents strong oxidation ability confirmed by the NH3 oxidation results (Fig. S6). However, it is interesting that the hierarchical pore structure of SAPO34-H promotes the formation of uniform Cu+ and Cu2+. Schneider et al. has demonstrated that Cu2+ and Cu+ are in favor of NH3-SCR reaction by DFT calculation and x-ray absorption spectroscopy, the NO and O2 are adsorbed on Cu+, and Cu+ was oxidized to Cu2+, while NH3 is beneficial to Cu2+ reduction to Cu+ [72,73]. The CO-DRIFTS results (Fig. 8) are in accord with the XPS results (Table 1 and Fig. S10), the more Cu+ is obtained in 2.7 wt%Cu/SAPO-34-H compared with the

Fig. 8. DRIFTS of CO adsorption on 2.7 wt%Cu/SAPO-34 and 2.7 wt%Cu/ SAPO-34-H (Pretreatment conditions: 1000 ppm CO, N2 as balance, total gas flow rate 200 mL/min).

with NOx more easily than the NH3 adsorbed on Brønsted acid for both 2.7 wt%Cu/SAPO-34-H and 2.7 wt%Cu/SAPO-34, and the NH3 consumption on different acid sites for 2.7 wt%Cu/SAPO-34-H is apparently higher than that for 2.7 wt%Cu/SAPO-34. But it is noticed the peaks of NH3 on Brønsted acid site are not changed for 7 and 5 min for 2.7 wt%Cu/SAPO-34 and 2.7 wt%Cu/SAPO-34-H, respectively (Figs. 10 and 11), and then the band intensity of NH3 adsorbed on Brønsted acid site gradually decreases. Wang and co-workers reported that Brønsted acid sites can store a large number of NH3 which can migrate to the Lewis acid sites contributing SCR reaction [33]. Meanwhile, comparing with the activity results, it can be concluded that NOx is easy to react with the Lewis acid species for 2.7 wt%Cu/SAPO-34-H. 3.12. Effect of hierarchical pore structure on activity over 2.7 wt%Cu/ SAPO-34-H Among xCu/SAPO-34-H catalysts, 2.7 wt%Cu/SAPO-34-H presents excellent NH3-SCR activity, in which the NOx conversion (> 90%) is in a broad window from 150 °C to 480 °C, together with outstanding hydrothermal stability at low temperature (80 °C) and high temperature

Fig. 9. DRIFTS of NH3 adsorption on 2.7 wt%Cu/SAPO-34 (A) and 2.7 wt%Cu/SAPO-34-H (B) at 130 °C (Pretreatment conditions: 1000 ppm NH3, N2 as balance, total gas flow rate 200 mL/min). 9

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Fig. 10. DRIFTS of 2.7 wt%Cu/SAPO-34 (A) and 2.7 wt%Cu/SAPO-34-H (B) in a flow of NO + O2 and pre-adsorbed NH3 at 130 °C (Pretreatment conditions: 1000 ppm NH3, 1000 ppm NO, 3% O2, N2 as balance, total gas flow rate 200 mL/min).

4. Conclusion xCu/SAPO-34-H (x = 1.1 wt%, 1.9 wt%, 2.7 wt% and 3.2 wt%) are prepared by ion-exchanged method using SAPO-34-H with hierarchical pore structure. Meanwhile the 2.7 wt%Cu/SAPO-34-H demonstrates excellent NH3-SCR performance in a broad temperature window from 150 °C to 470 °C. Moreover, the hydrothermal aging (80, 700 and 800 °C) and introducing H2O/SO2 have no obvious effect on NOx conversion for 2.7 wt%Cu/SAPO-34-H. Comparing with 2.7 wt%Cu/SAPO34 prepared using the traditional SAPO-34 as support, the 2.7 wt%Cu/ SAPO-34-H shows the moderate Cu+ and Cu2+ amounts in D6R of SAPO-34-H due to the special hierarchical pore structure which is beneficial to NH3-SCR reaction. Furthermore, in situ DRIFTS results illustrate that the E-R reaction mechanism occurs over 2.7 wt%Cu/SAPO34-H at 200 °C. It can be concluded that hierarchical pore structure in SAPO-34 plays an important role in the NH3-SCR of NOx. Acknowledgements Fig. 11. NH3 consumption on different acid sites with time in DRIFTS of the reaction between NOx and adsorbed NH3 over 2.7 wt%Cu/SAPO-34 and 2.7 wt %Cu/SAPO-34-H.

This study was supported by Harbin Science and Technology Innovation Talent Fund (Outstanding Academic Leader Project) (RC2016XK015004), Program from Heilongjiang Human Resources And Social Security Bureau, Postdoctoral Science Research Developmental Foundation of Heilongjiang Province of China (LBHZ16176), University Nursing Program for Young Scholars with Creative talents (UNPYSCT-2017115), Open Project fund from State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, and Youth Science and Technology Innovation Team Project of Heilongjiang Province (RCYJTD201803).

2.7 wt%Cu/SAPO-34 catalyst. In addition, the location of the copper species in SAPO-34 also plays an important role in the NH3-SCR reaction. As presented in EPR (Fig. 7), the Cu2+ species exists in the CHA cavity along the three-fold for the 2.7 wt%Cu/SAPO-34 catalyst, while Cu2+ primarily locates in D6R for 2.7 wt%Cu/SAPO-34-H. The literatures have been verified that the activity of Cu2+ in D6R is better than the activity of Cu2+ in hexagonal prism [32,63,64,74]. The above results indicate that hierarchical pore structure is beneficial to generate active Cu2+ species, which can explain the excellent performance of the 2.7 wt%Cu/SAPO-34-H catalyst. The generation of hierarchical pore structure can promote the formation of the moderate Cu+ and Cu2+ amounts in D6R of SAPO-34 which is beneficial to NH3-SCR reaction. Hence, 2.7 wt%Cu/SAPO-34-H presents excellent NH3-SCR activity. In situ DRIFTS results illustrate that the generation of hierarchal pore promotes the decrease of Brønsted acid sites, which is due to the changes of Si and Al contents. Meanwhile, from Fig. 11, it can be concluded that the NOx is facile to react with the adsorbed NH3 on Lewis acid sites for 2.7 wt%Cu/SAPO34-H.

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122376. References [1] E. Fridell, M. Skoglundh, B. Westerberg, S. Johansson, G. Smedler, NOx storage in barium-containing catalysts, J. Catal. 183 (1999) 196–209. [2] G. Centi, S. Perathoner, Nature of active species in copper-based catalysts and their chemistry of transformation of nitrogen oxides, Appl. Catal. A: Gen. 132 (1995) 179–259. [3] G. Busca, L. Lietti, G. Ramis, F. Berti, Chemical and mechanistic aspects of the selective catalytic reduction of NOx by ammonia over oxide catalysts: a review, Appl. Catal. B: Environ. 18 (1998) 1–36.

10

Chemical Engineering Journal 379 (2020) 122376

R. Li, et al.

[31] Y. Cao, D. Fan, P. Tian, L. Cao, T.T. Sun, S.T. Xu, M. Yang, Z.M. Liu, The influence of low-temperature hydration methods on the stability of Cu-SAPO-34 SCR catalyst, Chem. Eng. J. 354 (2018) 85–92. [32] J. Wang, T. Yu, X.Q. Wang, G.S. Qi, J.J. Xue, M.Q. Shen, W. Li, The influence of silicon on the catalytic properties of Cu/SAPO-34 for NOx reduction by ammoniaSCR, Appl. Catal. B: Environ. 127 (2012) 137–147. [33] D. Wang, L. Zhang, K. Kamasamudram, W.S. Epling, In situ-DRIFTS study of selective catalytic reduction of NOx by NH3 over Cu-exchanged SAPO-34, ACS Catal. 3 (2013) 871–881. [34] J.X. Liu, F.H. Yu, J. Liu, Y.C. Wei, Q.Y. Sun, Synthesis and kinetics investigation of meso-microporous Cu-SAPO-34 catalysts for the selective catalytic reduction of NO with ammonia, J. Environ. Sci. 48 (2016) 45–58. [35] T. Zhang, F. Qiu, J.H. Li, Design and synthesis of core-shell structured meso-Cu-SSZ13 mesoporous aluminosilicate catalyst for SCR of NOx with NH3: enhancement of activity, hydrothermal stability and propene poisoning resistance, Appl. Catal. B: Environ. 195 (2016) 48–58. [36] B. Arstad, A. Lind, J.H. Cavka, K. Thorshaug, D. Akporiaye, D. Wragg, H. Fjellvsag, A. Grønvold, T. Fuglerud, Structural changes in SAPO-34 due to hydrothermal treatment. A NMR, XRD, and DRIFTS study, Micropor. Mesopor. Mater. 225 (2016) 421–431. [37] X.M. Guo, D.S. Mao, S. Wang, G.S. Wu, G.Z. Lu, Combustion synthesis of CuO-ZnOZrO2 catalysts for the hydrogenation of carbon dioxide to methanol, Catal. Commun. 10 (2009) 1661–1664. [38] J. Baneshi, M. Haghighi, N. Jodeiri, M. Abdollahifar, H. Ajamein, Homogeneous precipitation synthesis of CuO–ZrO2–CeO2–Al2O3 nanocatalyst used in hydrogen production via methanol steam reforming for fuel cell applications, Energy Convers. Manage. 87 (2014) 928–937. [39] C.S. Blackwell, R.L. Patton, Aluminum-27 and phosphorus-31 nuclear magnetic resonance studies of aluminophosphate molecular sieves, J. Phys. Chem. 88 (1984) 6135–6139. [40] P.J. Barrie, J. Klinowsk, Ordering in the framework of a magnesium aluminophosphate molecular sieve, J. Phys. Chem. 93 (1989) 5972–5974. [41] R. Li, Z.B. Li, L.Q. Chen, Y.L. Dong, S.B. Ma, F.L. Yuan, Y.J. Zhu, Synthesis of MnNiSAPO-34 by a one-pot hydrothermal method and its excellent performance for the selective catalytic reduction of NO by NH3, Catal. Sci. Technol. 7 (2017) 4984–4995. [42] P.N.R. Vennestrøm, A. Katerinopoulou, R.R. Tiruvalam, A. Kustov, P.G. Moses, P. Concepcion, A. Corma, Migration of Cu ions in SAPO-34 and its impact on selective catalytic reduction of NOx with NH3, ACS Catal. 3 (2013) 2158–2161. [43] R. Vomscheid, M. Briend, M.J. Peltre, P.P. Man, D. Barthomeuf, The role of the template in directing the Si distribution in SAPO zeolites, J. Phys. Chem. 98 (1994) 9614–9618. [44] J. Tan, Z.M. Liu, X.H. Bao, X.C. Liu, X.W. Han, C.Q. He, R.S. Zhai, Crystallization and Si incorporation mechanisms of SAPO-34, Micropor. Mesopor. Mater. 53 (2002) 97–108. [45] W.L. Shen, X. Li, Y.X. Wei, P. Tian, F. Deng, X.W. Han, X.H. Bao, A study of the acidity of SAPO-34 by solid-state NMR spectroscopy, Micropor. Mesopor. Mater. 158 (2012) 19–25. [46] D. Barthomeuf, Topological model for the compared acidity of SAPOs and SiAl zeolites, Zeolites 14 (1994) 394–401. [47] X.T. Wang, R. Li, S.H. Bakhtiar, F.L. Yuan, Z.B. Li, Y.J. Zhu, Excellent catalytic performance for methanol to olefins over SAPO-34 synthesized by controlling hydrothermal temperature, Catal. Commun. 108 (2018) 64–67. [48] L. Wang, J.R. Gaudet, W. Li, D. Weng, Migration of Cu species in Cu/SAPO-34 during hydrothermal aging, J. Catal. 306 (2013) 68–77. [49] S.A. Bates, A.A. Verma, C. Paolucc, A.A. Parekh, T. Anggara, A. Yezerets, W.F. Schneider, J.T. Miller, W.N. Delgass, F.H. Ribeiro, Identification of the active Cu site in standard selective catalytic reduction with ammonia on Cu-SSZ-13, J. Catal. 312 (2014) 87–97. [50] A. Shishkin, H. Kannisto, P.A. Carlsson, H. Härelind, M. Skoglundh, Synthesis and functionalization of SSZ-13 as an NH3-SCR catalyst, Catal. Sci. Technol. 4 (2014) 3917–3926. [51] N. Wilken, R. Nedyalkova, K. Kamasamudram, J.H. Li, N.W. Currier, R. Vedaiyan, A. Yezerets, L. Olsson, Investigation of the effect of accelerated hydrothermal aging on the Cu sites in a Cu-BEA catalyst for NH3-SCR applications, Top. Catal. 56 (2013) 317–322. [52] H. Li, C. Paolucci, I. Khurana, L.N. Wilcox, F. Goltl, J.D. Albarracin-Caballero, A.J. Shih, F.H. Ribeiro, R. Gounder, W.F. Schneider, Consequences of exchange-site heterogeneity and dynamics on the UV-visible spectrum of Cu-exchanged SSZ-13, Chem. Sci. 10 (2019) 2373–2384. [53] D. Wang, Y. Jangjou, Y. Liu, M.K. Sharma, J.Y. Luo, J.H. Li, K. Kamasamudram, W.S. Epling, A comparison of hydrothermal aging effects on NH3-SCR of NOx over Cu-SSZ-13 and Cu-SAPO-34 catalysts, Appl. Catal. B: Environ. 165 (2015) 438–445. [54] T. Yu, J. Wang, M.Q. Shen, W. Li, NH3-SCR over Cu/SAPO-34 catalysts with various acid contents and low Cu loading, Catal. Sci. Technol. 3 (2013) 3234–3241. [55] X.X. Wang, Y. Shi, S.J. Li, W. Li, Promotional synergistic effect of Cu and Nb doping on a novel Cu/Ti-Nb ternary oxide catalyst for the selective catalytic reduction of NOx with NH3, Appl. Catal. B: Environ. 220 (2018) 234–250. [56] X.X. Yao, L. Zhang, L.L. Li, L.C. Liu, Y. Cao, X. Dong, F. Gao, Y. Deng, C.J. Tang, Z. Chen, L. Dong, Y. Chen, Investigation of the structure, acidity, and catalytic performance of CuO/Ti0.95Ce0.05O2 catalyst for the selective catalytic reduction of NO by NH3 at low temperature, Appl. Catal. B: Environ. 150–151 (2014) 315–329. [57] Q.Q. Hu, J.Q. Huang, G.J. Lia, Y.B. Jiang, Ha. Lan, W. Guo, Y.G. Cao, Origin of the improved photocatalytic activity of Cu incorporated TiO2 for hydrogen generation from water, Appl. Surf. Sci. 382 (2016) 170–177. [58] D.S. Zhou, B. Li, Z.X. Ma, X.D. Huang, X.B. Zhang, H.S. Yang, Synergy between FeOX

[4] D. Wang, Y.J. Jou, Y. Liu, M.K. Sharma, J. Luo, J. Li, K. Kamasamudram, W.S. Epling, A comparison of hydrothermal aging effects on NH3-SCR of NOx over Cu-SSZ-13 and Cu-SAPO-34 catalysts, Appl. Catal. B: Environ. 165 (2015) 438–445. [5] J.H. Kwak, D. Tran, S.D. Burton, J. Szanyi, J.H. Lee, C.H.F. Peden, Effects of hydrothermal aging on NH3-SCR reaction over Cu/zeolites, J. Catal. 287 (2012) 203–209. [6] J. Li, H. Chang, L. Ma, J. Hao, R.T. Yang, Low-temperature selective catalytic reduction of NOx with NH3 over metal oxide and zeolite catalysts – a review, Catal. Today 175 (2011) 147–156. [7] C. Liu, L. Chen, J.H. Li, L. Ma, H. Arandiyan, Y. Du, J.Y. Xu, J.M. Hao, Enhancement of activity and sulfur resistance of CeO2 supported on TiO2-SiO2 for the selective catalytic reduction of NO by NH3, Environ. Sci. Technol. 46 (2012) 6182–6189. [8] M. Moliner, C. Franch, E. Palomares, M. Grill, A. Corma, Cu-SSZ-39, an active and hydrothermally stable catalyst for the selective catalytic reduction of NOx, Chem. Commun. 48 (2012) 8264–8266. [9] D.Q. Fan, J. Wang, T. Yu, J.Q. Wang, X.Q. Hu, M.Q. Shen, Catalytic deactivation mechanism research over Cu/SAPO-34 catalysts for NH3-SCR (I): the impact of 950 °C hydrothermal aging time, Chem. Eng. Sci. 176 (2018) 285–293. [10] M.H. Xu, J. Wang, T. Yu, J.Q. Wang, M.Q. Shen, New insight into Cu/SAPO-34 preparation procedure: impact of NH4-SAPO-34 on the structure and Cu distribution in Cu-SAPO-34 NH3-SCR catalysts, Appl. Catal. B: Environ. 220 (2018) 161–170. [11] O. Mihai, C.R. Widyastuti, S. Andonova, K. Kamasamudram, J.H. Li, S.Y. Joshi, N.W. Currier, A. Yezerets, L. Olsson, The effect of Cu-loading on different reactions involved in NH3-SCR over Cu-BEA catalysts, J. Catal. 311 (2014) 170–181. [12] N. Wilken, R. Nedyalkova, K. Kamasamudram, J.H. Li, N.W. Currier, R. Vedaiyan, A. Yezerets, L. Olsson, Investigation of the effect of accelerated hydrothermal aging on the Cu sites in a Cu-BEA catalyst for NH3-SCR applications, Top. Catal. 56 (2013) 317–322. [13] K. Leistner, O. Mihai, K. Wijayanti, A. Kumar, K. Kamasamudram, N.W. Currier, A. Yezerets, L. Olsson, Comparison of Cu/BEA, Cu/SSZ-13 and Cu/SAPO-34 for ammonia-SCR reactions, Catal. Today 258 (2015) 49–55. [14] M. Iwamoto, H. Furukawa, Y. Mine, F. Uemura, S. Mikuriya, S. Kagawa, Copper(II) ion-exchanged ZSM-5 zeolites as highly active catalysts for direct and continuous decomposition of nitrogen monoxide, Chem. Soc. Chem. Commun. (1986) 1272–1273. [15] P.N.R. Vennestrøm, T.V.W. Janssens, A. Kustov, M. Grill, A. Puig-Molina, L.F. Lundegaard, R. Tiruvalam, P. Concepción, A. Corma, Influence of lattice stability on hydrothermal deactivation of Cu-ZSM-5 and Cu-IM-5 zeolites for selective catalytic reduction of NOx by NH3, J. Catal. 309 (2014) 477–490. [16] L. Chen, X.X. Wang, Q.L. Cong, H.Y. Ma, S.J. Li, W. Li, Design of a hierarchical FeZSM-5 CeO2 catalyst and the enhanced performances for the selective catalytic reduction of NO with NH3, Chem. Eng. J. 369 (2019) 957–967. [17] Y. Xin, X. Wang, Q. Li, X.C. Ma, Y.X. Qi, L.R. Zheng, J.A. Anderson, Z.L. Zhang, The potential of Cu-SAPO-44 in the selective catalytic reduction of NOx with NH3, ChemCatChem 8 (2016) 3740–3745. [18] Y. Xin, N. Zhang, X. Wang, Q. Li, X.C. Ma, Y.X. Qi, L.R. Zheng, J.A. Anderson, Z.L. Zhang, Efficient synthesis of the Cu-SAPO-44 zeolite with excellent activity for selective catalytic reduction of NOx by NH3, Catal. Today 332 (2019) 35–41. [19] Y. Xin, Q. Li, Z.L. Zhang, Zeolitic materials for DeNOx selective catalytic reduction, ChemCatChem 10 (2018) 29–41. [20] J. Wang, D.Q. Fan, T. Yu, J.Q. Wang, T. Hao, X.Q. Hu, M.Q. Shen, W. Li, Improvement of low-temperature hydrothermal stability of Cu/SAPO-34 catalysts by Cu2+ species, J. Catal. 322 (2015) 84–90. [21] R. Martínez-Franco, M. Moliner, P. Concepcion, J.R. Thogersen, A. Corma, Synthesis, characterization and reactivity of high hydrothermally stable Cu-SAPO34 materials prepared by “one-pot” processes, J. Catal. 314 (2014) 73–82. [22] K.P. Xie, J.W. Woo, D. Bernin, A. Kumar, K. Kamasamudram, L. Olsson, Insights into hydrothermal aging of phosphorus-poisoned Cu-SSZ-13 for NH3-SCR, Appl. Catal. B: Environ. 241 (2019) 205–216. [23] A.M. Beale, I. Lezcano-Gonzalez, W.A. Slawinskic, D.S. Wraggc, Correction: correlation between cu ion migration behavior and deNOx activity in Cu-SSZ-13 for the standard NH3-SCR reaction, Chem. Commun. 55 (2019) 1667. [24] J. Fan, P. Ning, Y.C. Wang, Z.X. Song, X. Liu, H.M. Wang, J. Wang, L.Y. Wang, Q.L. Zhang, Significant promoting effect of Ce or La on the hydrothermal stability of Cu-SAPO-34 catalyst for NH3-SCR reaction, Chem. Eng. J. 369 (2019) 908–919. [25] T. Usui, Z.D. Liu, S. Ibe, J. Zhu, C. Anand, H. Igarashi, N. Onaya, Y. Sasaki, Y. Shiramata, T. Kusamoto, T. Wakihara, Improve the hydrothermal stability of CuSSZ-13 zeolite catalyst by loading a small amount of Ce, ACS Catal. 8 (2018) 9165–9173. [26] L.M. Ren, L.F. Zhu, C.G. Yang, Y.M. Chen, Q. Sun, H.Y. Zhang, C.J. Li, F. Nawaz, X.J. Meng, F.S. Xiao, Designed copper-amine complex as an efficient template for one-pot synthesis of Cu-SSZ-13 zeolite with excellent activity for selective catalytic reduction of NOx by NH3, Chem. Commun. 47 (2011) 9789–9791. [27] U. Deka, I. Lezcano-Gonzalez, B.M. Weckhuysen, A.M. Beale, Local environment and nature of Cu active sites in zeolite-based catalysts for the selective catalytic reduction of NOx, ACS Catal. 3 (2013) 413–427. [28] R.D. Zhang, N. Liu, Z.G. Lei, B. Chen, Selective transformation of various nitrogencontaining exhaust gases toward N2 over zeolite catalysts, Chem. Rev. 116 (2016) 3658–3721. [29] P.S. Hammershøi, A.D. Jensen, T.V.W. Janssens, Impact of SO2-poisoning over the lifetime of a Cu-CHA catalyst for NH3-SCR, Appl. Catal. B: Environ. 238 (2018) 104–110. [30] S.Y. Joshi, A. Kumar, J.Y. Luo, K. Kamasamudram, N.W. Currier, A. Yezerets, New insights into the mechanism of NH3-SCR over Cu- and Fe-zeolite catalyst: apparent negative activation energy at high temperature and catalyst unit design consequences, Appl. Catal. B: Environ. 226 (2018) 565–574.

11

Chemical Engineering Journal 379 (2020) 122376

R. Li, et al.

[59]

[60]

[61]

[62] [63]

[64]

[65] [66]

[67] H. Sjövall, R.J. Blint, L. Olsson, Detailed kinetic modeling of NH3 SCR over Cu-ZSM5, Appl. Catal. B: Environ. 92 (2009) 138–153. [68] L.Q. Chen, R. Li, Z.B. Li, F.L. Yuan, X.Y. Niu, Y.J. Zhu, Effect of Ni doping in NixMn1xTi10(x=0.1-0.5) on activity and SO2 resistance for NH3-SCR of NO studied with in situ DRIFTS, Catal. Sci. Technol. 7 (2017) 3243–3257. [69] Z.P. Zhang, L.Q. Chen, Z.B. Li, P.Y. Li, F.L. Yuan, X.Y. Niu, Y.J. Zhu, Activity and SO2 resistance of amorphous CeaTiOx catalysts for the selective catalytic reduction of NO with NH3: in situ DRIFT studies, Catal. Sci. Technol. 6 (2016) 7151–7162. [70] L.Q. Chen, X.Y. Niu, Z.B. Li, Y.L. Dong, Z.P. Zhang, F.L. Yuan, Y.J. Zhu, Promoting catalytic performances of Ni-Mn spinel for NH3-SCR by treatment with SO2 and H2O, Catal. Commun. 85 (2016) 48–51. [71] K. Wijayanti, S. Andonova, A. Kumar, J.H. Li, K. Kamasamudram, N.W. Currier, A. Yezerets, L. Olsson, Impact of sulfur oxide on NH3-SCR over Cu-SAPO-34, Appl. Catal. B: Environ. 166–167 (2015) 568–579. [72] C. Paolucci, I. Khurana, A.A. Parekh, S. Li, A.J. Shih, H. Li, J.R. Di Iorio, J.D.A. Caballero, A. Yezerets, J.T. Miller, W.N. Delgass, F.H. Ribeiro, W.F. Schneider, R. Gounder, Dynamic multinuclear sites formed by mobilized copper ions in NOx selective catalytic reduction, Science 357 (2017) 898–903. [73] C. Paolucci, A.A. Verma, S.A. Bates, V.F. Kispersky, J.T. Miller, R. Gounder, W.N. Delgass, F.H. Ribeiro, W.F. Schneider, Isolation of the copper redox steps in the standard selective catalytic reduction on Cu-SSZ-13, Angew. Chem. Int. Ed. 53 (2014) 11828–11833. [74] L.J. Xie, F.D. Liu, L.M. Ren, X.Y. Shi, F.S. Xiao, H. He, Excellent performance of onepot synthesized Cu-SSZ-13 catalyst for the selective catalytic reduction of NOx with NH3, Environ. Sci. Technol. 48 (2014) 566–572.

and CuOX in FeCuOX/TiO2-hBN composites for NO reduction with NH3, J. Mol. Catal. A Chem. 409 (2015) 183–190. B. Li, Z.Y. Ren, Z.X. Ma, X.D. Huang, F. Liu, X.B. Zhang, H.S. Yang, Selective catalytic reduction of NO by NH3 over CuO-CeO2 in the presence of SO2, Catal. Sci. Technol. 6 (2016) 1719–1725. S. Ali, L.Q. Chen, F.L. Yuan, R. Li, T.R. Zhang, S.H. Bakhtiar, X.S. Leng, X.Y. Niu, Y.J. Zhu, Synergistic effect between copper and cerium on the performance of CuxCe0.5-x-Zr0.5(x = 0.1 − 0.5) oxides catalysts for selective catalytic reduction of NO with ammonia, Appl. Catal. B: Environ. 210 (2017) 223–234. S. Ali, L.Q. Chen, Z.B. Li, T.R. Zhang, R. Li, S.H. Bakhtiar, X.S. Leng, F.L. Yuan, X.Y. Niu, Y.J. Zhu, Cux-Nb1.1-x(x = 0.45, 0.35, 0.25, 0.15) bimetal oxides catalysts for the low temperature selective catalytic reduction of NO with NH3, Appl. Catal. B: Environ. 236 (2018) 25–35. Y.J. Kim, J.K. Lee, K.M. Min, S.B. Hong, I. Nam, B.K. Cho, Hydrothermal stability of CuSSZ13 for reducing NOx by NH3, J. Catal. 311 (2014) 447–457. M. Zamadies, X. Chen, L. Kevan, Study of Cu(II) location and adsorbate interaction in CuH-SAPO-34 molecular sieve by electron spin resonance and electron spin echo modulation spectroscopies, J. Phys. Chem. 96 (1992) 2652–2657. M. Zamadics, X. Chen, L. Kevan, Solid-state ion exchange in H-SAPO-34: electron spin resonance and electron spin echo modulation studies of copper (II) location and adsorbate interaction, J. Phys. Chem. 96 (1992) 5488–5491. P.E. Fanning, M.A. Vannice, A DRIFTS study of Cu-ZSM-5 prior to and during its use for N2O decomposition, J. Catal. 25 (2002) 166–182. K. Hadjiivanov, H. Knözinger, FTIR study of CO and NO adsorption and coadsorption on a Cu/SiO2 catalyst: probing the oxidation state of copper, Phys. Chem. Chem. Phys. 3 (2001) 1132–1137.

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