Irregular influence of alkali metals on Cu-SAPO-34 catalyst for selective catalytic reduction of NOx with ammonia

Journal of Hazardous Materials 387 (2020) 122007

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Irregular influence of alkali metals on Cu-SAPO-34 catalyst for selective catalytic reduction of NOx with ammonia

T

Guangpeng Yanga,b, Xuesen Dua,*, Jingyu Rana,*, Xiangmin Wanga, Yanrong Chena, Li Zhanga, Vladislav Racc, Vesna Rakicc, John Crittendenb a

Key Laboratory of Low-Grade Energy Utilization Technologies and Systems, School of Energy and Power Engineering, Chongqing University, Chongqing, 400044, China Brook Byers Institute for Sustainable Systems and School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA, 30332, United States c Faculty of Agriculture, Department of Chemistry, University of Belgrade, Nemanjina 6, 11080, Zemun, Serbia b

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

A R T I C LE I N FO

A B S T R A C T

Editor: Xiaohong Guan

SCR activity of Cu-SAPO-34 catalyst was reduced by alkali metal ions. The alkali metals ions (Li+, Na+ and K+) have shown irregular influences on Cu-SAPO-34. The order of poisoning strengths under 400 °C was found to be: Na+ > K+ > Li+, which is not consistent with the basicities of their corresponding metals. Experimental results and calculations showed that the alkali metal ions readily replace H+ and Cu2+/Cu+ ions. These exchanges result in the loss of Brønsted acid sites and migration of isolated Cu2+ ions in Cu-SAPO-34, which decrease the NH3-SCR activity. Both the basicity and ion diameter will affect the exchanging behavior of an alkali ion. Na+ and Li+ ions will influence both H+ and Cu2+/Cu+ ions but K+ ions only preferably replace the H+. We hypothesize that K+ cannot enter into a small ring (6-membered ring) to replace a Cu2+/Cu+ ion because of its large ion diameter. The displaced Cu2+/Cu+ ions will transfer to adjacent unbonded Al site to form a CuAlO2 species.

Keywords: Cu-SAPO-34 Alkali metal Migration Cu species

1. Introduction The selective catalytic reduction of NOx with NH3 (NH3-SCR) is an effective way to reduce the NOx emission from both mobile and point

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sources. The Cu-SAPO-34 catalyst used for removing NOx in mobile sources has drawn much attention, because of its high destruction efficiency for NOx reduction over a wide temperature range and superior resistance to hydrothermal aging, compared with traditional V-based

Corresponding authors. E-mail addresses: [email protected] (G. Yang), [email protected] (X. Du), [email protected] (J. Ran).

https://doi.org/10.1016/j.jhazmat.2019.122007 Received 24 September 2019; Received in revised form 7 December 2019; Accepted 30 December 2019 Available online 31 December 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

Journal of Hazardous Materials 387 (2020) 122007

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plane copper oxide clusters was responsible for the deactivation (Ma et al., 2015). However, Wang et al. studied the impact of potassium on Cu-SAPO-34 catalysts and found that the copper species remained unchanged. Wang et al. found that Brønsted acidity decreased with the increasing potassium loadings, which was responsible for the reduced NH3-SCR activity of Cu-K-SAPO-34 catalysts (Wang et al., 2015). Based on previous literatures, the interaction between alkali metals and reactive sites of Cu-SAPO-34 catalyst, especially at the molecular level, remain unknown or debatable. As a result, a systematically investigation on the impacts of alkali metals on Cu-SAPO-34 catalyst is needed. To investigate the influences of alkali metal poisoning on Cu-SAPO34, alkali metals (Li, Na, and K) were impregnated into the fresh CuSAPO-34 catalysts with 1.48 wt.% and 2.25 wt.% copper loadings. The influences of alkali metal ions on the crystalline structures, acid sites and the nature of Cu species in the Cu-SAPO-34 are discussed. It should be clear that Li does not exist in the real flue gas and we are studying it for comparison to discuss the poisoning mechanism. The X-ray diffraction was conducted to investigate the influence of alkali metals on the crystalline structures. We also conducted NH3-TPD, H2-TPR, and EPR tests, and collected UV-vis-DRS and XAFS spectra to investigate the poisoning behavior of alkali metals. In addition, DFT calculations were performed to determine the locations of alkali metals and the impacts of alkali metal ions on Cu species at molecular level.

catalysts (Fickel et al., 2011; Li et al., 2011; Beale et al., 2015; Niu et al., 2016; Wang et al., 2018a,b; Wang et al., 2020). Urea solution is regarded as a harmless NH3 precursor, which has been used for the reduction of NOx in diesel engine vehicles. However, the urea solution and a variety of additives in engine lubrication oils contain a certain amount of alkali metal ions (e.g. K+, Na+) (Kröcher and Elsener, 2008; Nicosia et al., 2008). These metal impurities deposit on the catalyst, resulting in a significant deactivation of the catalyst (Fan et al., 2018; Wang et al., 2018c; Lezcano-Gonzalez et al., 2014). The interaction between alkali metal ions and the reactive sites is the reason for the deactivation of NH3-SCR catalysts. Some researchers observed that Cu species mainly existed as Cu2+ ions located in the surface of six-membered ring (6MR) or CHA cage (Wang et al., 2012; Deka et al., 2012; Bates et al., 2014; Xue et al., 2013; Gao et al., 2013a; Shen et al., 2015). Mao et al. reported that 6MR is the most favorable position for isolated Cu2+ ions, which coordinated to three lattice oxygen atoms based on the density functional theory (DFT) calculation results (Mao et al., 2016). Isolated Cu2+ ions in 6MR were identified as active center for NH3-SCR reaction at low temperature range (Deka et al., 2012; Bates et al., 2014; Wang et al., 2014; Paolucci et al., 2016; Wang et al., 2017; Gao et al., 2013b). Meanwhile, Zhang et al. reported the existence of Cu-OH species in Cu-SSZ-13 catalyst (Zhang et al., 2014). Jangjou et al. detected Cu-OH species in Cu-SAPO-34 catalyst based on IR spectra, which is supposed to locate near eight-membered ring (8MR) (Jangjou et al., 2016). Recent studies have concluded that whether copper species tends to located in the 6MR as isolated Cu ions or in the cage near 8MR as Cu-OH species primarily depends on the Si/ Al ratio (Xue et al., 2013; Wang et al., 2014; Paolucci et al., 2014; Gao et al., 2015a). Gao et al. and Paolucci et al. reported that copper existed in the SSZ-13 zeolite as Cu2+ or Cu-OH, and Cu-OH also makes a significant contribution to the NH3-SCR reaction rate (Paolucci et al., 2016; Gao et al., 2017). Besides, Cu-O-Cu species were detected in CuCHA catalysts with high copper loadings (Gao et al., 2013a; Wang et al., 2013; Leistner and Olsson, 2015; Zhao et al., 2017). Therefore, to investigate the interaction between the deposited alkali metals and these copper species is required to understand the deactivation mechanism of Cu-SAPO-34 catalyst. Poisoning effects of alkali metals on traditional metal oxides catalysts had been thoroughly investigated. The lowered surface acidity and reducibility were the primary reasons for the deactivation (Du et al., 2012a,b; Du et al., 2015). The effects of alkali metal ions on Cu-SSZ-13 catalyst are completely dependent on the introduction methods. Zhao et al. proposed that Na+ improved both the low-temperature activity and hydrothermal stability of Cu-SSZ-13 catalysts using a two-step ion exchange method. The framework Al was preserved after hydrothermal treatment and the reducibility of Cu ion in 6MR was enhanced by Na+ ions (Zhao et al., 2017; Gao et al., 2015b). Gao et al. reported other alkali metal ions (Li+, K+ and Cs+) maintained the framework of CuSSZ-13 during hydrothermal aging (Gao et al., 2015b). Fan et al. observed the Cu-SSZ-13 catalysts were deactivated after the introduction of alkali metals (Na, K, Mg and Ca) using an incipient wetness impregnation method. The deactivation resulted from the transformation of Cu2+ to abundant extra-framework CuO clusters and pore blocking (Fan et al., 2018). Cu-SAPO-34 catalysts showed a higher resistance to alkali metal poisoning than traditional V-W/Ti oxide catalysts (Liu et al., 2015). Ma et al. investigated the effect of impregnated potassium on isolated Cu2+ ions and the Brønsted acid sites in Cu-SAPO-34 catalysts, and concluded that transformation of isolated Cu2+ to square-

2. Experimental and computational details 2.1. Catalysts preparation and aging treatment A two-step liquid ion-exchange method was used to prepare the CuSAPO-34 catalyst. Firstly, the NH4-SAPO-34 was prepared by exchanging H-SAPO-34 powder (Nankai University Catalyst Co., Ltd., Al:Si:P = 1:0.25:0.75) with a 0.1 M NH4NO3 (Alfa Aesar, > 95 %) solution at 80 °C for 2 h. The ratio of weight of powder (g) to volume of solution (ml) is 1:50. Then, the solid was separated from the mixture and washed with deionized water several times. The ammonium-exchange process was carried out twice. Secondly, Cu-SAPO-34 was obtained by mixing the NH4-SAPO-34 with a Cu(CH3COO)2 (Aladdin Co., 99.0 %) solution at 80 ᵒC for 2 h. After the exchange process, the powder was dried at 110 ᵒC for overnight, and calcined at 550 ᵒC for 4 h with a ramping rate of 10 ᵒC min−1. The loadings of Cu were determined using ICP and the results are shown in Table 1. The alkali-contained Cu-SAPO-34 samples were prepared using an incipient wetness impregnation method with M (M = lithium, sodium and potassium) nitrate solution. These samples were dried at 110 ᵒC for overnight and calcined at 550 ᵒC for 4 h. The mole ratio of copper to the alkali metal was 1:1. As a control, the alkali-free samples were impregnated with deionized water and otherwise treated like the alkalicontained samples. The fresh Cu-SAPO-34 catalysts were labeled as xCu-F, where “x” represents the content of Cu, and the aged samples were labeled as xCu-yM, where “y” represents the content of alkali metals. The alkali metal loadings were listed in Table 1. 2.2. NH3-SCR activity measurement The NH3-SCR activity was measured in a quartz reactor using 0.2 g sample (40–60 mesh) at atmospheric pressure. The feed gas composition was 500 ppm NH3, 500 ppm NO, 5 % O2 and N2 balance. The total gas flow rate was 1000 ml/min and the weight hourly space velocity

Table 1 The composition of fresh and alkali metals impregnated Cu-SAPO-34 catalysts and the concentration of Cu(CH3COO)2 solutions. Sample

Concentration of Cu(CH3COO)2 (mol/L)

Cu content (wt.%)

Li content (wt.%)

Na content (wt.%)

K content (wt.%)

1.48Cu-M 2.25Cu-M

0.00625 0.01875

1.48 2.25

0.16 0.24

0.53 0.81

0.91 1.73

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(WHSV) was equal to 300,000 ml g−1 h−1. The effluent gas composition was analyzed online with a Fourier Transform infrared (FTIR) spectrometer, after the temperatures and outlet concentrations were stable. The reaction temperature was elevated from 100 ᵒC to 500 ᵒC, and the NO conversion was calculated using the following Eq. (1):

NO conversion [%] =

NOinlet − NOoutlet 100% NOinlet

operated at energy of 3.5 GeV and a current between 150−210 mA (Yu et al., 2015). Ultraviolet-visible (UV–vis) spectra experiments were performed to examine the Cu species. The spectra were collected at room temperature using a Shimadzu UV-3600 spectrometer in the range of 10000−50000 cm−1 wavelength.

(1) 2.4. The kinetic measurements

2.3. Catalysts characterization NH3-SCR kinetic tests were performed in a quartz reactor using 25 mg catalyst. The use of small particles sizes (60–80 mesh) and a high gas flow rate (1.5 l min−1, WHSV = 3,600,000 ml g−1 h−1) eliminated the impact of internal and external diffusion. The influent concentration was 500 ppm NO, 500 ppm NH3, 5 % O2 with N2 as the balance gas. Kinetic steady-state measurements were made from 160 ᵒC to 260 ᵒC, and NH3-SCR reaction rates (mol NO gcatal s−1) was calculated from NO conversion by Eq. (2):

The powder X-ray diffraction (XRD) patterns were performed using a Bruker D8 Advance instrument with nickel-filtered Cu Kα radiation (λ = 1.5418 Å). The XRD pattern was measured in the range of 5ᵒ < 2θ < 50ᵒ with a step size of 0.026ᵒ at room temperature. The temperature programmed reduction with H2 (H2-TPR) experiments were performed to evaluate the reducibility and distribution of Cu species in catalysts. 0.2 g sample was placed in a quartz tube reactor and pretreated at 500 ᵒC in N2 prior to the experiments. In the H2-TPR experiment, the temperature was elevated from 100 to 800 ᵒC at a ramping rate of 10 ᵒC min−1 with a flow of 10 vol.% H2/N2 (50 ml min−1). In order to quantify the amount of each copper species, silver oxide (Ag2O, Aladdin Co., 99 %) was used to calibrate the thermal conductivity detector (TCD) signals. Temperature Programmed Desorption with NH3 (NH3-TPD) experiments were performed to evaluate the acid sites of catalysts. A 0.2 g sample was placed in a quartz tube reactor and pretreated at 500 ᵒC for 30 min in N2 before the experiments, and then cooled to 30 ᵒC. NH3 adsorption was carried out with 500 ppm NH3/N2 until the outlet NH3 concentration was stable. Then the catalysts were heated to 100 ᵒC and purged with N2 to remove weakly adsorbed NH3 until the outlet signal remained stable. Finally, the temperature was elevated from 100 ᵒC to 600 ᵒC at a ramping rate of 10 ᵒC min−1 for these samples. The electron paramagnetic resonance (EPR) spectra experiment was performed to examine the coordination environment and qualify the content of isolated Cu2+ ions. The spectra were acquired on a Bruker A300 instrument. The samples were pretreated at vacuum 120 ᵒC in air for 4 h before the tests. Then X-band (υ = 9.78 GHz) EPR spectra were obtained at −120 ᵒC, and the magnetic field was swept from 2000 to 4000 G. g factors were calculated by equation: hυ = gβH, where h is the Planck’s constant, υ is the frequency of the applied electromagnetic wave, β is the Bohr magneton, and H is the magnetic field. The commercial Origin software was used for the double integration of EPR spectra. The X-ray absorption data at the Cu K-edge of the samples were recorded at room temperature in transmission mode using ion chambers or in the fluorescent mode with silicon drift fluorescence detector at beam line BL14W1 of the Shanghai Synchrotron Radiation Facility (SSRF), China. The station was operated with a Si (111) double crystal monochromator. During the measurement, the synchrotron was

reaction rate = −

ln(1−XNO) × FNO m catal × 60 × 22.4

(2)

XNO = NO conversion (%), FNO = volumetric flow rate of NO (L(NO) min−1), mcatal (g) = mass of catalysts, and 22.4 (L/mol) = gas volume at standard temperature and pressure (273 oC and 1 atm).

2.5. Computational details The supercell DFT calculation were performed using CASTEP code in Materials Studio 8.0 (Clark et al., 2005; Fischer, 2015). The PBE function was used to describe the electron exchange and correlation (Fischer, 2015; McEwen et al., 2012; Halasz et al., 2015; Li et al., 2016; Du et al., 2018). A plane wave cut off energy was 400 eV, and the spin polarized was considered in all calculations. The self-consistent-field (SCF) tolerance energy was converged to 10−6 eV/atom, and the configurations was regarded as fully relaxed when the force was less than 0.05 eV/Å. The Brillouin zone was sampled at the Γ point for insulation (Paolucci et al., 2016). The periodic structures of Cu-SAPO-34 zeolite with different copper species and acid sites were built in the 2 × 1 × 1 supercells. These periodic models containing Cu+, Cu2+, [CuIIOH]+ species and lattice Bronsted acid sites were given in our previous research (Yang et al., 2018a). The adsorption energy (Eads) is calculated using Eq. (3):

Eads = Emolecular + zeolite − Emolecular − Ezeolite

(3)

Where the Emolecular+zeolite, Ezeolite and Emolecular are the total energies of the zeolite with the adsorbed molecule, the clean zeolite system, and the isolated molecular in the gas phase, respectively. Fig. 1. XRD patterns of the fresh and alkali metal poisoned Cu-SAPO-34 catalysts with different Cu contents of (a) 1.48 wt.% and (b) 2.25 wt.%. The fresh Cu-SAPO-34 catalysts are labeled as xCu-F, where “x” represents the content of Cu, and the aged samples are labeled as xCu-yM (M = Li, Na and K), where “y” represents the content of alkali metals. This labeling will be applied throughout the paper.

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Fig. 2. NO conversions over fresh and poisoned (a) 1.48Cu-SAPO-34 samples, and (b) 2.25Cu-SAPO-34 samples. The feed gas composition was 500 ppm NH3, 500 ppm NO, 5 % O2 and N2 balance.

3. Results

results, the low-temperature peak at around 180 °C was assigned to NH3 desorption from weak acid sites, and these sites are likely extra framework Si or Si−OH sites (Wang et al., 2014; Gao et al., 2015b; Wang et al., 2015). The peaks at around 280 and 405 °C were attributed to NH3 desorption from Lewis acid sites created by isolated Cu and Cu-OH species, and strongly bonded NH3 on the Brønsted acid sites, respectively (Wang et al., 2014, 2015; Ma et al., 2014b). By determining the relative amounts of NH3 desorption, all alkali-contained samples showed reduced NH3 desorption intensity from Brønsted acid sites in Fig. 3c and d. This result indicated some H+ sites were substituted by Li+, Na+ and K+ ions. In addition, for Li- and K-contained samples, NH3 desorption intensity at around 280 °C was somewhat enhanced. These results again suggested that substitution between Li+ and K+ with H+ occurred and some new sites were created by Li+ and K+. No significant increase on NH3 desorption intensity at around 280 °C was observed for Na-contained samples. As we can see from Fig. 3b, the intensity of 280 °C ammonia desorption peak obviously decreased for the 2.25Cu-0.81Na sample, which strongly indicated the loss of acid sites created by Cu species. Therefore, we supposed that compared with Li+ and K+ ions, Na+ ions are more likely to affect Cu species and reduce the number of Lewis acid sites.

3.1. The influence of alkali metals on the crystalline morphology of CuSAPO-34 As shown in Fig. 1, typical CHA structures peak features were exhibited in all samples, indicating that the framework structures were maintained well after alkali metals impregnation (Wang et al., 2013). There were no diffraction peaks assigned to CuO at 35.3° and 38.5°in both fresh and poisoned catalysts, suggesting no extra-framework CuO was formed in the Cu-SAPO-34 after alkali metals impregnation (Niu et al., 2016). 3.2. The influence of alkali metals on the NH3-SCR activity of Cu-SAPO-34 NO conversions of fresh and poisoned samples with various Cu and alkali metal contents are shown in Fig. 2. Accordingly, high Cu loadings catalysts performed better at low temperatures, especially at 200–250 ᵒC. At high temperatures over 400 ᵒC, NO conversions decreased slightly with the increase of Cu loadings. All poisoned samples showed the inhibition on NO conversions in the whole temperature range. It should be noted that the decrease of catalytic activity by Na for all samples was the most severe at the temperature range of 150–400 ᵒC. Moreover, Li shows more pronounced activity inhibition than K at the temperature range of 175–225 ᵒC for 1.48 wt.% Cu loading samples. Besides, both 1.48 wt.% and 2.25 wt.% Cu loading Li-contained samples showed lower NO conversion than K-contained samples at 425–500 ᵒC. As a result, the extent alkali metals poisoning effect on the Cu-SAPO-34 catalysts under 400 ᵒC can be ordered as: Na > K > Li. These results indicate the deactivation is not related to the basicity of alkali metals, when the mole ratio of Cu to alkali metal is 1:1. Isolated Cu2+ ions were proposed as active center for NH3-SCR at low temperatures, and the Bronsted acid sites made great contribution to the catalytic activity at high temperatures (Wang et al., 2012, 2015; Zhang et al., 2016; Ma et al., 2014a). Therefore, the deactivation over the entire temperature may be related to the reduction of acid sites and the decease of active copper species.

3.3.2. DFT calculations for NH3 adsorption Based on the NH3-TPD results and our previous works (Yang et al., 2018a, b), different acid sites including Cu-OH species, isolated Cu2+ species and Brønsted acid sites (lattice OH) were investigated by DFT calculations to identify NH3 adsorption sites. The adsorption energies and optimized Cu-SAPO-34 local structures with interatomic distances are shown in Fig. 4. The local structures for NH3 adsorption on alkali metals (Li, Na and K) exchanged Cu-SAPO-34 catalysts are shown in Fig. S2. The adsorption energy for NH3 adsorption on the -OH site of Cu-OH species was only −0.51 eV, indicating this site is a weak Brønsted acid site. Adsorbed NH3 molecule on Cu site of Cu-OH and isolated Cu2+ displayed moderate adsorption energies of −1.04 and −1.28 eV, respectively. In addition, NH3 adsorption energies on two lattice -OH sites (lattice OH1 and lattice OH2) were much high (−1.34 eV and 1.76 eV, respectively), which indicated that lattice -OH sites are strong Brønsted acid sites. These calculation results are consistent with NH3-TPD curves of Cu-SAPO-34 catalysts. Table 2 showed that the substitutions between alkali metal ions and lattice H+ are strongly exothermic (K > Na > Li). Fig. 4 exhibited that the NH3 adsorption on these alkali metal ions were obviously weaker than that on the lattice -OH sites. These results indicated that the amount of adsorbed NH3 on Brønsted acid sites can be easily reduced by alkali metal ions. Moreover, NH3 adsorption energy on isolated Cu2+ site was remarkably reduced from −1.28 eV to −0.91 eV, when Na ions were located at the nearby 8MR. Combining the calculational and experimental results, we suggested Na ions tended to

3.3. The influence of alkali metals on NH3 adsorption profile of Cu-SAPO34 3.3.1. NH3-TPD results Fig. S1 presented the NH3-TPD curve of the fresh H-SAPO-34 sample, and two NH3 desorption peaks at around 180 and 405 °C were observed. In Fig. 3a and b, a new NH3 desorption peak (B) developed at around 280 °C for Cu-SAPO-34, and the intensity of peak B increased with the increasing Cu loading. This is likely due to NH3 desorption from Cu species. According to the early reports and our experimental 4

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Fig. 3. NH3 temperature-programmed desorption (NH3-TPD) curves for the fresh and poisoned (a) 1.48Cu-SAPO-34 and (b) 2.25CuSAPO-34 samples, and relative amounts of the acid sites in the fresh and poisoned (c) 1.48CuSAPO-34 and (d) 2.25Cu-SAPO-34 samples based on NH3-TPD. NH3 desorption peaks at around 180, 280 and 405 °C were labeled as A, B and C.

reduce NH3 adsorption onto Cu2+ species.

Table 2 The relatively energies for the substitution between alkali metal ions and H+ site.

3.4. Copper species and their interaction with alkali metal ions in the CuSAPO-34

CuOH → CuOM Lattice OH1 → Lattice OM1 Lattice OH2 → Lattice OM2

As shown in Fig. S3a, fresh samples with different Cu loadings showed two H2 reduction peaks at around 210 and 270 °C, respectively. In Fig. S5, H2-TPR curve for CuO-SAPO-34 catalyst prepared via impregnation method showed that reduction temperature of CuO clusters is lower than that of isolated Cu2+. Based on these curves and early reports (Wang et al., 2012; Xue et al., 2013; Liu et al., 2015; Wang et al., 2015; Ma et al., 2013), the peak at around 210 ℃ was assigned to the one-step reduction of CuO clusters to Cu°, and another peak at around 270 ℃ was attributed to the reduction of isolated Cu2+ ions. Meanwhile, for the fresh 1.48Cu-SAPO-34 samples, another two peaks at the temperature of 582 and 670 °C represented the reduction of lowstability Cu+ (L-Cu+) and high-stability Cu+ (H-Cu+) ions to Cu°, respectively (Niu et al., 2016; Fan et al., 2018; Liu et al., 2015). As shown in Figs. 6 and S10, 6MR (site 1) is the most stable position for isolated

Li (eV)

Na (eV)

K (eV)

−1.26 −2.31 −2.39

−1.08 −2.65 −2.95

−1.74 −3.26 −3.70

Cu2+ ions, according to the relative energies listed in Table S1. However, when copper loadings increased from 0.98 wt.% to 1.73 wt.%, extra Cu ions moved to the window of 8MR (site 2) or to the one Sisubstituted 6MR (site 1) as original Cu+ species. As a result, the L-Cu+ and H-Cu+ was generated by the reduction of Cu+ located in site 2 and site 1, respectively. The reduction temperature of Cu+ ions in site 1 was higher than that in site 2 because of the steric hindrance (Fan et al., 2018; Chen et al., 2018). It should be noticed that only the fresh 0.98Cu-SAPO-34 sample (Fig. S3) showed an incomplete peak at Fig. 4. Energy profiles for NH3 adsorption on the acid sites created by copper species and alkali metal ions, and Brønsted acid sites (M = H, Li, Na and K). Local structures for NH3 adsorption on pure Cu-SAPO-34 models are included in the figure. O in lattice OH1 belongs to a 4membered ring (4MR) and an 8-membered ring (8MR), and O in lattice OH2 belongs to a 6-membered ring (6MR) and an 8-membered ring. Green, red, purple, yellow, orange, blue and white balls represent P, O, Al, Si, Cu, N, and H atoms, respectively. This arrangement will be applied throughout the paper (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

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Fig. 5. H2-TPR profiles of the fresh and poisoned (a)1.48Cu-SAPO-34 and (b) 2.25Cu-SAPO-34 samples, and the amounts of each copper species in the fresh and poisoned (c) 1.48Cu-SAPO-34 and (d) 2.25Cu-SAPO-34 samples based on H2-TPR. The relative amounts of isolated Cu2+ and CuAlO2 species are labeled in the figure.

around 785 ℃, which represented the extremely stable Cu+ ions generated by the reduction of isolated Cu2+ ions in site 1 (Fan et al., 2018; Wang et al., 2015; Zhang et al., 2016). Therefore, the reduction temperature of these extremely stable Cu+ species can be more than 800 ℃. In Fig. S4, the amount of Cu2+ species increased slightly when copper content increased from 1.73 wt.% to 2.25 wt.%, indicating that site 1 was almost fully occupied. Meanwhile, Cu+ peaks disappeared and a new peak at around 485 °C appeared. In our previous study, it was found that a single copper ion tended to make a pair with another one to form Cu-O-Cu complexes, which was located near the window of 8MR (site 2) (Yang et al., 2018b). Therefore, it was reasonable to propose that the H2 consumption peak at around 485 °C was due to the reduction of the Cu-O-Cu complexes. The transformation of copper species was obvious after the alkali metal poisoning. As shown in Fig. 5a, the H2 consumption peak of HCu+ species shifted from 670 °C to a lower temperature (600−630 °C), and the amount of this Cu+ species decreased significantly in Li- and Na-contained samples. Such a phenomenon was probably because that

the interaction between Cu+ species and the framework can be easier reduced by Li+ and Na+ ions, rather than K+ ions. Meanwhile, the H2 consumption peak assigned to the reduction of Cu-O-Cu complexes appeared at around 475−493 °C for alkali metal impregnated samples. We supposed that the slight aggregation of copper species resulted from the distribution of alkali metal ions. The reduced H2 consumption peak of Cu-O-Cu complexes was observed at around 362−374 °C for the poisoned 2.25 wt.% Cu loading samples, indicating that the interaction between Cu-O-Cu complexes and the frameworks was weakened. Additionally, Wang et al. proposed that more Cu-O-Cu complexes are formed in Cu-SAPO-34 catalyst in the presence of potassium, which is consistent with our results in Fig. 5 (Wang et al., 2015). The calculated relatively binding energies for alkali metal ions in different sites were listed in Table S1. Site 1 is the most favorable position for Li+ and Na+ ions, while K+ ions tends to locate in site 2. Therefore, it further proved that more Cu-O-Cu complexes were produced when these sites were occupied by alkali metal ions. Moreover, a new H2 consumption peak appeared at around 365 °C

Fig. 6. Structure diagram of the unit cell of SAPO-34. All sites are labeled in the potential positions for copper species and alkali metal ions. 6

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Fig. 7. EPR spectra of the fresh and poisoned (a) 1.48Cu-SAPO-34 samples, and (b) 2.25Cu-SAPO-34 samples collected at −120 °C.

The shoulder B at ∼8986.2 eV was assigned to 1s → 4p transition in four-fold or three-fold coordinated Cu2+ sites, which can be also observed for CuO reference (Fig. S8) (Kau et al. (1989); Martini et al. (2018)). A high-intensity feature C indicating highly coordinated Cu2+ was present at ∼8995.3 eV, where Cu2+ is saturated by water molecules or by a combination of framework oxygens and water molecules (Giordanino et al., 2014; Alayon et al., 2013). For alkali metal poisoned samples, the intensity of feature C was reduced with respect to fresh samples, especially for Na. Meanwhile, a slight enhancement of the intensity of feature A was observed, as shown in Fig. S8 with a magnification of peak A. These differences were expected due to the migration of highly coordinated Cu2+ ions to extra-framework sites (CuAlO2 species), which is consistent with H2-TPR and EPR results. In addition, the FT-EXAFS spectra in Fig. S8e and f showed a slightly increase of the first coordination shell for deactivated samples, accompanied by the modifications of the signal in the 2–3 Å region, which indicated the decreased interaction of Cu ion with the framework after the addition of alkali metal ions.

(1.48Cu-M-SAPO-34) and 305 °C (2.25Cu-M-SAPO-34) can be attributed to CuAlO2 species near site 2 (Zhao et al., 2017; Chen et al., 2018; Deka et al., 2013). Fig. 5c and d showed that the more CuAlO2 species was generated in Li- and Na-contained samples than that in K-contained sample. Meanwhile, the amount of isolated Cu2+ species in 6MR decreased most significantly in the Na-contained samples. Additionally, the amounts of CuO species slightly increased for alkali metal impregnated samples. UV-vis-DRS spectra shown in Fig. S6 confirmed the migration of Cu species and indicated Na+ and Li+ have more significant influence on Cu species than K+. Therefore, we supposed that Li+ and Na+ ions could preferably cause the migration of Cu2+/Cu+ species. Fig. 7 showed the EPR profiles of the fresh and poisoned Cu-SAPO34 samples. All samples showed the same g// = 2.372 and g⊥ = 2.044 assigned to the characteristic signal peaks of isolated Cu2+ species located in site 1 (Shen et al., 2015). Four hyperfine splitting peaks related to the substance type and coordination information were clearly observed from the spectra of all samples. The strength of these characteristic peaks for poisoned samples declined. Fig. S7 showed the relatively amounts of isolated Cu2+ ions calculated by double integrating the EPR signals. These results indicated the reduction of the amounts of active isolated Cu2+ species, especially for Na-contained samples, which is consistent with the H2-TPR results (Xue et al., 2013). Combined with H2-TPR results, it is reasonable to suggest that isolated Cu2+ ions in alkali metal impregnated samples migrated to form CuAlO2 species, which is EPR silent. Fig. 8 reported the XANES and FT-EXAFS spectra of fresh and poisoned Cu-SAPO-34 samples in their hydrated state, which was collected at room temperature. A weak pre-edge peak A was clearly observed at ∼8976.1 eV. This transition was conventionally assigned to 1 s→3d transition in Cu2+ (Sano et al., 1992; Llabrés i Xamena et al., 2003).

4. Discussion As shown in Figs. 2 and S9, NH3-SCR activity increased at the low temperature of 150−300 °C with the increase of the copper loading from 1.48 to 2.25 wt.%. This is due to the increase of active Cu2+ species. In the low temperature range, the NH3-SCR reaction begins with the adsorption of NH3 on Cu2+ species, and then the formation of NH2NO intermediate (Mao et al., 2016; Paolucci et al., 2014; Yang et al., 2018b). After the alkali metal impregnation, low-temperature activity decreased, and Na+ ions exhibited the most severe inhibition on the NH3-SCR activity at the temperature range of 150–400 ᵒC. H2TPR and EPR results showed the greatest loss for the number of Cu2+

Fig. 8. Cu K-edge XANES spectra of fresh and poisoned (a) 1.48Cu-SAPO-34, and (b) 2.25Cu-SAPO-34 samples. All samples were collected at room temperature in the hydrated state. The principal XANES features are labeled with the A–C letters, and the insets show magnifications of highly coordinated Cu2+ peak (C). 7

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Fig. 9. RateNO of NH3-SCR reaction over fresh and poisoned (a) 1.48Cu-SAPO-34 and (b)2.25Cu-SAPO-34 catalysts as a function of reaction temperature. Fig. 10. Energy diagrams of Cu ion migration from the plane of 6MR to the adjacent 8MR in the presence of alkali metal ions (Li+, Na+ and K+). Alkali metal ions located in the bottom of double 6MR cage and Cu ion in the plane of 6MR in state A. Na+ and K+ ions entered to the middle of the 6MR cage in state B. Alkali metal ions located in the plane of 6MR, while Cu ion in the adjacent 8MR in state C. Local structures are shown in Fig. S11.

Fig. 11. The mechanism of the poisoning behavior of different alkali metal on Cu-SAPO-34 catalyst.

fresh and poisoned samples have similar Ea (∼44 kJ/mol for 1.48 wt.% Cu loaded samples and 42 kJ/mol for 2.25 wt.%), indicating alkali metal ions hardly change the reaction mechanism on isolated Cu2+ sites at around 200 °C (Xue et al., 2013; Chen et al., 2018). Besides, the reaction rates varied directly with the amount of Cu2+. As shown in Fig. S12, there is no noticeable difference between the fresh and alkalicontained samples on the N2O concentration at the temperature range of 100–400 ᵒC. Alkali metals can hardly degrade the N2 selectivity of

species in Na-contained samples among all poisoned Cu-SAPO-34 catalysts. A contrary tendency of activity for Li- and K-contained samples at different Cu loading was shown in Fig. 2 at the temperature of 200 °C. The difference can be assigned to the different numbers of Cu2+ species in Li- and K-contained samples shown in Figs. 5 and S7. These results indicated that the number of Lewis acid Cu2+ sites primarily limited the activity of Cu-SAPO-34 catalyst at 200 °C. Fig. 9 showed the reaction rates with respect to 1000/T over fresh and poisoned Cu-SAPO-34. The

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Cu-SAPO-34 catalysts under 400 ᵒC. More N2O was generated on poisoned samples than the fresh ones Over 400 ᵒC, indicating that alkali metals can slightly reduce the N2 selectivity at high temperature range. At the temperature range of 250−350 °C, 1.48Cu-0.91 K sample displayed a lower NO conversion than 1.48Cu-0.16Li sample. Previous studies showed that NH2NO decomposition to H2O and N2 on the Brønsted acid sites was relatively easier than on a Cu2+ site by a hydrogen push-pull mechanism (Mao et al., 2016; Brüggemann and Keil, 2008). Calculated energies in Table 2 showed that H+ sites can be earlier substituted by K+ ions than by Li+ ions, and NH3-TPD results demonstrated more severe loss of Brønsted acid sites in K-contained samples. As a result, NH3 coverage on the Brønsted acid sites was relate to the NH3-SCR activity. Some new sites created by Li+ and K+ increased the NH3 desorption intensity at around 280 °C, but the activities decreased. We evaluated NH3 decomposition process on these Li and K sites using DFT method, as we reported that NH3 decomposition to NH2 and H was the first step of the reduction reaction for NH3-SCR in our previous work (Yang et al., 2018b). NH3 needed to overcome an energy barrier of 1.50 eV to decompose to NH2 and H on Cu2+ site. Fig. S13 showed that the energy barriers of NH3 decomposition on Li and K sites are 4.66 and 5.13 eV, respectively, and NH3 and NO can hardly coadsorb on Li and K sites. As a result, we supposed that the adsorbed NH3 on these additional adsorption sites (Li+ and K+) can hardly participate in the SCR process. Thus, the factors affecting activity were the number of active Cu2+ species and Brønsted acid sites. H2-TPR and EPR results showed that after alkali poisoning the isolated Cu2+ ions in 6MR migrated to form CuAlO2 species. Therefore, the migration of isolated Cu ions from the 6MR to the adjacent 8MR was evaluated. The energy diagrams and local structures were shown in Figs. 10 and S11, respectively. In the presence of Na+ ions, the migration of isolated Cu2+ ions needed to overcome a relatively lower energy barrier (1.08 eV) than that of Li+ ion (1.80 eV) and K+ ion (1.86 eV). Li+ ion showed strong interaction with lattice O atoms, which caused the high energy barrier for the migration of Cu and Li ions. Whereas, Table S1 showed that K+ preferred to enter 8MR rather than 6MR. The energy barrier for the migration of K+ ion from the CHA cage to the interior of double 6MRs was 0.80 eV, which is because that the diameter of K+ (2.76 Å) is much larger than that of Li+ (1.52 Å) and Na+ (2.04 Å) but smaller than that of the pore window of CHA (8MR, 3.8 Å). Figs. 5 and S7 showed that more isolated Cu2+ site in 2.25 wt.% Cu loading catalysts were reduced by Na+ and K+ than those in 1.48 wt.% Cu loading catalysts. In the 1.48 wt.% Cu loading catalyst, both Cu+ and Cu2+ exist due to the rich defect sites and well distribution of Cu ions. When Na+ or K+ are introduced into the catalyst, the Cu+ or Cu2+ ions are both severely influenced. But in the 2.25 wt.% Cu loading catalyst, only Cu2+ exists, either in the form of isolated Cu2+/6MR or Cu-O-Cu, as indicated in Fig. 5. Consequently, the influence of Na+ and K+ on Cu2+ seem to be more significantly for 2.25 wt.% Cu loading catalyst than 1.48 wt.% Cu loading catalyst, which promoted the migration of Cu2+ ions. These results demonstrated that Cu2+ ions were substituted by alkali metal ions and migrated to inactive sites. As a result, the interaction mechanism of different alkali metals with Cu-SAPO-34 catalyst was shown in Fig. 11. The migration of isolated Cu2+ ions to form CuAlO2 species resulted in the reduced NH3-SCR activity over the whole temperature range. Besides, the deactivation of Cu-SAPO-34 catalyst above the temperature of 250 °C could not rule out the reduction of the Brønsted acid sites. Li+ and Na+ ions had the same preferable site with Cu ions, and the diameter and basicity of alkali metal ions affected the substitution between alkali metal ions and isolated Cu ions.

stronger influence than K and Li on the SCR performance of Cu-SAPO34 catalyst, which is different from the poisoning profile of transition metal oxide catalyst that more basic metal will cause more severe poisoning influence. Two main reasons were found for the poisoning consequence. First, H2-TPR, UV-vis, EPR and XAFS results demonstrated that alkali ions will compete with Cu2+ to coordinate with the oxygen atoms of 6MR, leading to the migration of Cu2+ out from 6MR to form CuAlO2 species in adjacent cages. Second, NH3-TPD curves and DFT calculations indicated that Brønsted acid sites were eliminated by alkali metal ions, which is detrimental to the activities above 250 °C. The results also showed that K+ imposed more influence on the decrease of acidity and more framework Si-O(H+)-Al sites were exchanged by K+. DFT calculations confirmed that the energy barrier for the migration of Cu2+ out from 6MR after interacting with Na+ is lower than those after interacting with K+ and Li+. Thus, Na+ is more ready to replace Cu2+ to coordinate in 6MR. Combined with experimental and computational methods, the poisoning behavior was revealed to be related to both basicity and the ion diameter of alkali metals. CRediT authorship contribution statement Guangpeng Yang: Conceptualization, Methodology, Writing - original draft. Xuesen Du: Investigation, Writing - original draft, Supervision. Jingyu Ran: Supervision. Xiangmin Wang: Software, Visualization. Yanrong Chen: Resources. Li Zhang: Project administration. Vladislav Rac: Writing - review & editing. Vesna Rakic: Writing - review & editing. John Crittenden: Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We gratefully acknowledge the financial support of National Natural Science Foundation of China (51506015), Chongqing Technology Innovation and Application Demonstration Projects (cstc2018jscxmsyb0999), Fundamental Research Funds for the Central Universities (2018CDQYDL0050, 2018CDJDDL0004), Open Fund of Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of Education of China (LLEUTS-2019002), and beamline BL14W1 (Shanghai Synchrotron Radiation Facility) for providing the beam time. The authors appreciate the support from the Brook Byers Institute for Sustainable Systems, Hightower Chair and Georgia Research Alliance at the Georgia Institute of Technology. The views and ideas expressed herein are solely those of the authors and do not represent the ideas of the funding agencies in any form. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.122007. References Alayon, E.M.C., Nachtegaal, M., Bodi, A., van Bokhoven, J.A., 2013. Reaction conditions of methane-to-methanol conversion affect the structure of active copper sites. ACS Catal. 4, 16–22. Bates, S.A., Verma, A.A., Paolucci, C., Parekh, A.A., Anggara, T., Yezerets, A., Schneider, W.F., Miller, J.T., Delgass, W.N., Ribeiro, F.H., 2014. Identification of the active Cu site in standard selective catalytic reduction with ammonia on Cu-SSZ-13. J. Catal. 312, 87–97. Beale, A.M., Gao, F., Lezcano-Gonzalez, I., Peden, C.H., Szanyi, J., 2015. Recent advances in automotive catalysis for NOx emission control by small-pore microporous materials. Chem. Soc. Rev. 44, 7371–7405.

5. Conclusion An irregular phenomenon regarding the poisoning of Cu-SAPO-34 catalyst by alkali metals has been found and studied. Na showed 9

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