Spectroscopic identification and catalytic relevance of NH4+ intermediates in selective NOx reduction over Cu-SSZ-13 zeolites

Chemosphere 250 (2020) 126272

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Spectroscopic identification and catalytic relevance of NHþ 4 intermediates in selective NOx reduction over Cu-SSZ-13 zeolites € rn Martin Tabak a, Jia-Yue Yang d, Valentina Rizzotto a, 1, Dongdong Chen b, c, 1, Bjo Daiqi Ye b, c, Ulrich Simon a, Peirong Chen a, b, c, * a

Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1a, 52074, Aachen, Germany Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, School of Environment and Energy, South China University of Technology, 510006, Guangzhou, China c National Engineering Laboratory for VOCs Pollution Control Technology and Equipment, 510006, Guangzhou, China d Optics & Thermal Radiation Research Center, Shandong University, 266237, Qingdao, China b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 The relative content of NHþ 4 intermediates was determined by in situ DRIFTS.  Cu redox cycle in NH3-SCR is closely related to the formation of NHþ 4 intermediates.  The catalytic relevance of NHþ 4 intermediates was elaborated.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 September 2019 Received in revised form 1 February 2020 Accepted 18 February 2020 Available online 20 February 2020

Reduction of harmful nitrogen oxides (NOx) from diesel engine exhausts is one of the key challenges in environmental protection, and can be achieved by NH3-assisted selective catalytic reduction (NH3-SCR) using copper-exchanged chabazite zeolites (i.e. Cu-CHA, including Cu-SSZ-13 and Cu-SAPO-34) as catalysts. Understanding the redox chemistry of Cu-CHA in NH3-SCR catalysis is crucial for further improving the NOx reduction efficiency. Here, a series of Cu-SSZ-13 catalysts with different Cu ion exchange levels were prepared, thoroughly characterized by different techniques such as X-ray diffraction, diffuse reflectance ultravioletevisible spectroscopy and temperature-programmed desorption using NH3 as a probe molecule, etc., and tested in NH3-SCR reactions under steady-state conditions. In situ studies by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), supplemented with densityfunctional theory calculations, provided solid evidence for the formation of ammonium ion (NHþ 4 ) intermediates resulting from the reduction of Cu2þ to Cuþ by co-adsorbed NH3 and NO molecules on CuSSZ-13. Catalytic relevance of the NHþ 4 intermediates, as demonstrated by an increase of NO conversion over Cu-SSZ-13 pre-treated in NH3/NO atmosphere, can be attributed to the formation of closely coupled þ Cuþ/NHþ 4 pairs promoting the Cu re-oxidation and, consequently, the overall NH3-SCR process. This study thus paves a new route for improving the NH3-SCR efficiency over Cu-CHA zeolite catalyst. © 2020 Elsevier Ltd. All rights reserved.

Handling Editor: Peizhe Sun Keywords: In situ DRIFTS Cu redox Nitrogen oxides Zeolite catalyst DFT calculation

* Corresponding author. School of Environment and Energy, South China University of Technology, 510006, Guangzhou, China. E-mail address: [email protected] (P. Chen). 1 These authors contributed equally. https://doi.org/10.1016/j.chemosphere.2020.126272 0045-6535/© 2020 Elsevier Ltd. All rights reserved.

1. Introduction Copper-exchanged small-pore zeolites have been widely used as

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catalysts in NH3-assisted selective catalytic reduction (NH3-SCR) of nitrogen oxide (NOx), owing to their excellent catalytic properties and hydrothermal stability (Brandenbergeret al., 2008; Kwak et al., 2010; Granger and Parvulescu, 2011). Among others, Cu-SSZ-13, a typical Cu-exchanged chabazite (CHA) zeolite catalyst, has been utilized and commercialized successfully in the removal of NOx in automotive diesel exhausts (Borfecchia et al., 2018; Marberger et al., 2018). In the past decade, significant progress has been achieved in understanding the structure-activity relationship and NH3-SCR mechanism of Cu-SSZ-13 (Godiksen et al., 2017; Chen et al., 2019a, 2019b), which has also advanced the mechanistic understanding of Cu-zeolite NH3-SCR catalysts in general (Lomachenko et al., 2016; Paolucci et al., 2017). Brønsted acid sites, generated by the framework substitution of Si by Al in SSZ-13 and the subsequent charge compensation, play a vital role in Cu ion-exchange by influencing the siting and local structure of the introduced Cu species (including Cu2þ and [CuOH]þ) (Bates et al., 2014; Paolucci et al., 2014; Di Iorio et al., 2015; Gao et al., 2015). According to Paolucci et al., in standard NH3-SCR reaction (i.e. 4NO þ 4NH3 þ O2 / 4N2 þ 6H2O), the reduction of Cu2þ in the six-member ring (6 MR) generates an intermediate proton on the proximal Brønsted acid site (Paolucci et al., 2016). The newly formed proton interacts with adsorbed NH3 on metal site to form a highly mobile NHþ 4 intermediate (Coq et al., 2004; Bates et al., 2014; Paolucci et al., 2014, 2016). In our recent investigations, formation of NHþ 4 intermediates resulted from the reduction of Cu2þ to Cuþ in the presence of NH3 and NO was directly reflected by in situ electrical impedance spectroscopy in combination with diffuse reflectance infrared Fourier transform spectroscopy (in situ IS-DRIFTS) (Chen et al., 2016a, 2016b, 2016c, 2018). Although the Cu2þ 4 Cuþ redox cycle and associated formation of NHþ 4 intermediates have been confirmed repeatedly in different studies (Doronkin et al., 2014; Günter et al., 2015; Janssens et al., 2015; Chen et al., 2016a, 2016b, 2016c, 2018; Zhang et al., 2019), until now it is still not clear how the in situ formed NHþ 4 intermediates are related to the Cu redox cycle and the NH3SCR performance of Cu-SSZ-13. In standard NH3-SCR reactions, the mobility of Cu species, in particular the inter-cage Cu motion, is decisive for the reactivity of Cu-SSZ-13 at low temperatures (<250  C) (Paolucci et al., 2017). It is well accepted that the re-oxidation of Cuþ in NO/O2 atmosphere is the rate-determining step of the NH3-SCR reaction (Paolucci et al., 2014, 2016). In our previous investigation, we carried out densityfunctional theory (DFT) calculations over a Cu-SAPO-34 zeolite system to understand the role of NHþ 4 intermediates in NH3-SCR, and found that the NHþ 4 intermediates could couple closely with Cuþ to form Cuþ/NHþ 4 pairs which then promote the formation of þ Cu2þNO2/NHþ 4 , i.e. the re-oxidation of Cu , via reacting with NO/O2. These preliminary DFT results suggest that NHþ 4 intermediates play a very positive role in the Cu redox chemistry and are conducive to a promoted NH3-SCR activity of Cu-CHA zeolite catalyst (Chen et al., 2018). In this contribution, Cu-SSZ-13 catalysts with different Cu/Al ratios were synthesized and evaluated in standard NH3SCR catalysis. In situ DRIFTS was applied to determine the relative content of in situ generated NHþ 4 intermediates over the synthesized Cu-SSZ-13 catalysts by comparing the change of band intensity at 1460 cm1. Catalytic tests over Cu-SSZ-13 pretreated in NO/NH3 atmosphere confirmed that the NH3SCR efficiency was promoted by the formation of NHþ 4 intermediates. On the basis of the experimental and DFT calculation results, the catalytic relevance of NHþ 4 intermediates in the standard NH3-SCR reaction was elaborated.

2. Materials and methods 2.1. Synthesis Proton-form H-SSZ-13 (CHA-type framework, Si/Al ¼ 13) and Cu(NO3)2$3H2O were used as obtained. The synthetic procedure for H-SSZ-13 was reported elsewhere (Clemens et al., 2015). For the preparation of Cu-SSZ-13 samples, 3.5 g of H-SSZ-13 zeolite and different dosage of Cu(NO3)2$3H2O were added into a 1 L flask containing 120 mL ultra-pure water. The solution was stirred for 24 h at room temperature, or at 80  C under reflux in order to increase the Cu loading. The solid product was recovered by filtration, washed with ultra-pure water (a total of 1 L) and dried at 110  C overnight. The whole preparation process (ion exchange at 80  C, washing and drying) was repeated in order to achieve an even higher Cu loading. The zeolite products after drying were ground into fine powders and calcined at 500  C for 2 h leading to Cu-SSZ13 zeolite catalysts. The nomenclature of the different SSZ-13 samples is based on the Cu/Al ratio determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES), as Cu-xxSSZ-13 (xx represents the Cu/Al ratio). 2.2. Characterization The H- and Cu-SSZ-13 zeolites were characterized by ICP-OES, scanning electron microscopy (SEM), X-ray diffraction (XRD), diffuse reflectance ultraviolet/visible spectroscopy (DR UV/Vis) and temperature-programmed desorption using NH3 as a probe molecule (NH3-TPD). For ICP-OES analysis, the solid zeolite samples were dissolved in HF and subsequently analysed with an Analytical Instruments Model Spectro device. SEM measurements were carried out using a LEO/ZEISS Supra 35 VP microscope equipped with a Gemini column and a field emission electron. XRD patterns were recorded using a powder diffractometer (STADI P, Stoe & Cie GmbH) armed with a Cu-anode as X-ray source, which generates Cu Ka1 irradiation (1.54059 Å), and with a focussing germanium monochromator. DR UV/Vis spectra were collected with a PerkinElmer Lambda 650 spectrometer equipped with a Praying Mantis mirror system, and BaSO4 was used as reference. 50 mg of zeolites were introduced into a Harrick high-temperature reaction chamber, which was connected to a gas dosing system. A first spectrum under N2 (100 mL/min) at 30  C was recorded for comparison. Afterwards, the zeolite powders were activated at 500  C in a flow of pure O2 atmosphere (100 mL/min). A second spectrum was recorded as soon as the reaction chamber with zeolite was cooled down to 30  C again. Each spectrum was recorded with a data interval of 2 nm between 200 and 900 nm. For NH3-TPD measurements, 50 mg of zeolite sample was loaded into a quartz tube with an inner diameter of 6 mm and fixed with quartz wool. For pre-treatment, the zeolite catalyst was heated in O2 (with a flow rate of 50 mL/min) up to 500  C at a ramping rate of 7.5  C/min in a Carbolite tube furnace and held at the same temperature for 1 h. After that, the tube reactor was cooled down to 50  C in flowing N2. Following this, 1000 ppm of NH3 in N2 was fed to the catalyst for 2 h, succeeded by 3 h of flushing with pure N2. Then, the temperature was increased to 700  C at a ramping rate of 2  C/min in N2. The composition of outlet gas mixture was detected and analysed by an AAB LIMAS11 UV gas analyser placed downstream. 2.3. In situ DRIFTS The zeolite powder (ca. 30 mg) was placed inside a Harrick hightemperature reaction Chamber (HVC-DRP) for in situ DRIFTS measurements. The IR spectroscopy in diffuse reflection mode was

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measured using a FT-IR VERTEX 70 device (Bruker) with the help of a Harrick Praying Mantis mirror system. The zeolite catalyst was heated to 400  C and held at that temperature for 1 h in flowing O2 (with a flow rate of 50 mL/min). After that, the chamber with catalyst was cooled down to the respective measuring temperature in flowing N2 (50 mL/min). Each DRIFT spectrum was taken during a duration of 105 s and consisted of 128 scans. In a typical measurement, a spectrum in N2 was recorded as background and was subtracted from the spectra collected afterwards. A total of 21 spectra was measured in flowing NH3 (500 ppm in N2) before the atmosphere was changed to NO (500 ppm in N2). The atmosphere was changed to 10% O2 after another 20 spectra were recorded in NO. Due to practical reasons, H2O was not fed into the in situ reaction chamber in all the DRIFTS measurements. Band intensity at characteristic wavenumber was evaluated by integrating the respective peak area, and was normalized to [0, 1] to visualize the change of relative intensity in different atmospheres.

2.4. Computational details The first-principles DFT calculations were performed with the Vienna ab initio simulation package (VASP) (Kresse and Furthmüller, 1996) using the Perdew-Burke- Ernzehof (PBE) exchange-correlation functional. The Brillouin zone integration was computed with the 6  6  6 Monkhorst-Pack k-mesh and the cutoff energy was chosen as 400 eV. For structural optimization, the force convergence threshold was set as 5 meV/Å.

2.5. NH3-SCR catalysis One hundred milligram of zeolite catalyst was loaded into a quartz tube reactor with a diameter of 1 cm and fixed with quartz wool. For pre-treatment, the tube reactor with zeolite catalyst was heated in 50% O2 (with a flow rate of 200 mL/min, N2 balance) up to 500  C at a ramping rate of 7.5  C/min in a Carbolite tube furnace and held at the same temperature for 1 h. Immediately after the pre-treatment, the gas feed was changed to a mixture of NO/NH3/ O2 (250 ppm NO, 250 ppm NH3, 10% O2, balance N2) for NH3-SCR reaction at the same temperature of 500  C. Then, the temperature was lowered every 2 h to 425  C, 350  C, 300  C, 250  C, 200  C, 175  C, 150  C and 125  C in sequence, while the gas composition was kept the same. The outlet gas composition was analysed by an AAB LIMAS11 UV gas analyser. The NO conversion in NH3-SCR was calculated based on Eq. (1):

NO conversion ð%Þ ¼

Cinlet; NO  Coutlet;NO  100% Cinlet;NO

(1)

In order to examine the role of NHþ 4 intermediates in NH3-SCR catalysis, the catalytic reaction was also conducted at a fixed temperature of 200  C over Cu-SSZ-13 zeolite catalyst after exposure to a mixture of NO/NH3 for 2 h. Sample preparation and pre-treatment (500  C for 1 h in O2) were performed similarly as above mentioned. After pre-treatment, the furnace temperature was lowered to 200  C over the course of 5 h, and a mixture of NO/NH3 was feed to the catalyst bed for 2 h. The NH3-SCR reaction was initialled by adding O2 to the gas feed. A comparative measurement without exposure to NO/NH3 mixture was also conducted as reference. It has to be noted that H2O was also excluded from the gas feed in catalytic tests in order to establish a relationship of NHþ 4 intermediate formation and standard NH3-SCR performance over Cu-SSZ13 samples under comparable conditions.

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3. Results and discussion 3.1. Physicochemical characterization Cu-SSZ-13 catalysts with a Cu/Al ratio of 0.14, 0.18 and 0.24 were obtained and named as Cu-0.14-SSZ-13, Cu-0.18-SSZ-13 and Cu0.24-SSZ-13, respectively. Surface morphologies of the pristine and synthesized zeolites were investigated by SEM, and the representative images are shown in Fig. 1aed. It can be seen that the H- and Cu-SSZ-13 zeolites have very similar surface morphologies, suggesting the zeolite surface was not changed by the Cu ion exchange process. XRD was applied to investigate the crystal structure of the zeolites samples, and the results are shown in Fig. 1e. The Cu-SSZ13 samples exhibited identical characteristics as H-SSZ-13. Formation of large crystalline copper oxides was not observed in any of the Cu-SSZ-13 zeolites, suggesting the introduced Cu species were highly dispersed without detectable agglomerations. DR UVeVis was applied to further study the state of Cu species in the Cu-SSZ-13 samples. According to literature, Cu-SSZ-13 can be easily hydrated under ambient conditions, leading to watersolvated Cu ions that give rise to weak signal in UV/Vis (Fig. S1a) (Gao et al., 2013b; Negri et al., 2019). To exclude such interference, in situ DR UV/Vis measurements were performed, and the collected spectra after and before activation in O2 are shown in Fig. 2 and Fig. S1b, respectively. In theory, signals appearing in wavelengths below 250 nm are corresponding to the ligand metal charge transfer (LMCT) band related to isolated Cu2þ sites (i.e. O/Cu2þ). It can be seen clearly that the LMCT band intensity below 250 nm increases with the Cu/Al ratio due to a rising amount of Cu2þ in CuSSZ-13 (Korhonen et al., 2011; Deka et al., 2012; Lezcano-Gonzalez et al., 2014b). Interestingly, in Fig. 2, weak and broad bands were also noticed between 450 and 850 nm, which is attributed to Cu species with O-containing ligands (e.g. CuOx or [CueOeCu]2þ) (Bennici et al., 2003; Ipek et al., 2017; Hao et al., 2018). According to the Cu site compositional phase diagram predicted by Paolucci et al. (2016), both Cu2þ (see DFT-computed local structure in Fig. S2) and [CuOH]þ co-exist in the synthesized Cu-SSZ-13 catalysts. After high-temperature activation in O2, the [CuOH]þ species may be reduced partially and transformed to [CueOeCu]2þ, leading to the band at 450e850 nm (Smeets et al., 2005; Ipek et al., 2017). It has to be noted that in situ UV/Vis alone only provides limited information about the Cu state in the zeolite catalysts. A combination of different techniques (such as EPR, XAS, IR, etc.) is required to understand the exact coordination environment of the Cu sites (Godiksen et al., 2017; Gao et al., 2013a; Giordanino et al., 2013), which is not the focus of this work. A comparison of the recorded NH3 desorption profiles for the SSZ-13 samples is shown in Fig. 3a. The peaks at ca. 140  C, ca. 240  C and ca. 380  C represent the desorption of NH3 on Lewis sites (e.g. extra-framework Al sites) with a weak adsorption strength, NH3 on Cu sites with a medium adsorption strength and NHþ 4 on Brønsted sites with a strong adsorption strength, respectively (Wang et al., 2014, 2015; Gao et al., 2015). With increasing Cu/ Al ratio, the peak at ca. 240  C increased steadily, whereas that at ca. 380  C resulting from the desorption of NHþ 4 on Brønsted sites decreased, indicating that more Cu2þ ions have been incorporated into the zeolite by ion exchange with protons on Brønsted sites (Chen and Simon, 2016; Chen et al., 2018). With an increased amount of isolated Cu sites for NH3 adsorption, it is very likely that NHþ 4 (NH3)n complexes formed within the zeolite pore structure and consequently led to an increased amount of weakly bound NH3 species, as indicated by the increase of desorption peak at ca. 140  C. A further increase of Cu loading (Cu/Al ¼ 0.24) reduced significantly the available Brønsted sites (see the significantly lowered NH3 desorption at ca. 380  C), which is unfavorable for the formation of

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Fig. 1. SEM images of (a) H-SSZ-13, (b) Cu-0.14-SSZ-13, (c) Cu-0.18-SSZ-13 and (d) Cu-0.24-SSZ-13; (e) XRD patterns of H- and Cu-SSZ-13 samples.

NHþ 4 (NH3)n complexes. As a consequence, Cu-0.24-SSZ-13 exhibits a decreased desorption peak at ca. 140  C. Quantitative analysis of NH3 absorbed on Cu sites for all samples was conducted via peak deconvolution. As indicated by the results in Fig. 3b, while the interaction between Cu sites and NH3 remained similar in Cu-SSZ13 zeolites with low Cu/Al ratios (0.14 and 0.18) (Chen et al., 2018), less Cu active sites were available for NH3 adsorption due to the formation of oxo-dimeric [CueOeCu]2þ species at a high Cu loading (Cu/Al 0.24) (Giordanino et al., 2013; Grundner et al., 2015). 3.2. In situ DRIFTS studies

Fig. 2. In situ DR UVeVis spectra for Cu-SSZ-13 after activation in O2. The spectra were obtained after subtraction using the spectrum of H-SSZ-13 as background. Note the measurements were performed at 30  C in flowing O2.

In situ DRIFTS over Cu-0.14-SSZ-13 at different temperatures were conducted, and the spectra of zeolite catalyst after NH3 saturation and after subsequent exposure to NO are shown in Fig. 4a and b, respectively. At higher temperatures, the vibrational energy of NH3 increased, hindering its adsorption on zeolite and the formation of NHþ 4 (NH3)n complexes (Chen et al., 2016a). Therefore, with increasing temperature, the amount of NH3 adsorbed on zeolite decreased (Fig. 4a), as indicated by the decrease of band intensity at 3355 cm1 and 3273 cm1 for ammonium ions,

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Fig. 3. (a) NH3 desorption profiles of H- and Cu-SSZ-13 from NH3-TPD measurements; (b) integral peak area for the desorption of NH3 on Cu sites derived from the deconvolution of the NH3-TPD profiles in Fig. 3a.

Fig. 4. Comparison of normalized in situ DRIFT spectra for Cu-0.14-SSZ-13 at different temperatures. (a) after NH3 saturation; (b) after subsequent exposure in NO.

at 3181 cm1 for coordinated NH3 absorbed on Cu species (Ma et al., 2014; Xie et al., 2014), the band intensity for stretching vibrations of NH3 on Lewis sites at 1270 cm1 and at 1460 cm1 for NHþ 4 on Brønsted sites (Ma et al., 2014; Xie et al., 2014). After NO exposure, the amounts of adsorbed NH3 species decreased, especially for the weakly absorbed NH3 on Lewis sites. Surprisingly, the band intensity for NHþ 4 on Brønsted sites increased after NO exposure, €vall et al., indicating the generation of additional NHþ 4 species (Sjo 2006; Klukowski et al., 2009; Metkar et al., 2011; Gao et al., 2013a; Paolucci et al., 2014). This can be seen more clearly in Fig. S3, where the absolute change of band intensity at 1460 cm1, reflecting the relative content of NHþ 4 , is displayed during DRIFTS measurement at the selected temperature. As reported in literature and also in our previous studies (Paolucci et al., 2014, 2016; Chen et al., 2018), the increased band intensity at 1460 cm1, namely 2þ formation of NHþ to 4 intermediates, is due to the reduction of Cu Cuþ by co-adsorbed NH3 and NO via

Cu2þ þ NH3 þ NO/ Cuþ =Hþ þ H2 O þ N2

(2)

Hþ þ NH3 / NH4 þ

(3)

In situ DRIFTS over Cu-SSZ-13 with different Cu loadings were performed at 175  C, and the normalized spectra are shown in Fig. 5. Over the NH3-saturated zeolites (Fig. 5a), in general, there appears a decrease of band intensity for NHþ 4 on Brønsted sites (1460 cm1) (Ma et al., 2014; Xie et al., 2014) and an increase of band intensity for NH3 on Lewis sites (1270 cm1) along with the increase of copper loading (Lezcano-Gonzalez et al., 2014a), due to a consumption of Brønsted sites by the exchange of Cu2þ ions. On the contrary, bands at 1270 cm1, 1619 cm1 and 3400-3100 cm1 increased, which is attributed to [Cu(NH3)4]2þ complex resulting from NH3 adsorbed onto Cu sites (Williamson et al., 1975; Delabie et al., 2000; Lezcano-Gonzalez et al., 2014a). After subsequent NO exposure (Fig. 5b), the amount of NHþ 4 on Brønsted sites increased unexpectedly in all the Cu-SSZ-13 zeolites, suggesting the

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Fig. 5. Comparison of normalized spectra collected at 175  C (normalized to the highest band intensity in the respective spectrum) for H- and Cu-SSZ-13 samples. (a) after NH3 saturation; (b) after subsequent exposure in NO.

generation of NHþ 4 intermediates, whereas band intensities for other NH3 species decreased. The formation of NHþ 4 intermediates can be seen more clearly in Figs. S4 and S5, where the spectra after NH3 saturation and after subsequent NO exposure at 175  C are directly compared. The relative content of NHþ 4 intermediates can be reflected by the absolute change of band intensity at 1460 cm1 (Fig. S4), which indicates that the content of in situ generated NHþ 4 intermediates follows the order: Cu-0.18-SSZ-13 > Cu-0.24-SSZ13 > Cu-0.14-SSZ-13 > H-SSZ-13. It is known that Cu2þ ion in 6 MR can be reduced by NO and NH3, generating an intermediate proton on the adjacent Brønsted acid site and eventually leading to the formation of a NHþ 4 intermediate by further binding with a NH3 molecule on the nearby Cu site (Klukowski et al., 2009; Gao et al., 2013a; Giordanino et al., 2014; Lezcano-Gonzalez et al., 2014a; Chen et al., 2018; Zhang et al., 2019). At high Cu loadings, the isolated Cu2þ sites tend to form less reducible [CueOeCu]2þ dimers or even CuOx oligomers, as suggested by the DR UV/Vis (Fig. 2) and NH3-TPD (Fig. 3) studies. As a result, a lower amount of in situ generated NHþ 4 intermediates was observed in Cu-0.24-SSZ-13 than in Cu-0.18-SSZ-13. 3.3. NH3-SCR catalysis Catalytic performance of the Cu-SSZ-13 catalysts in NH3-SCR are shown in Fig. 6. NO conversions in the absence of any catalyst and over H-SSZ-13 are also included for comparison. The NO conversion of Cu-SSZ-13 samples (even with a low Cu/Al ratio of 0.14) are significantly higher than that of H-SSZ-13 at the respective temperature. Increasing the Cu/Al ratio of Cu-SSZ-13 to 0.18 led to the highest NH3-SCR activity in terms of NO conversion. However, the zeolite sample with the highest Cu/Al ratio, i.e. Cu-0.24-SSZ-13, shows a lower activity than Cu-0.18-SSZ-13, in particular at temperatures above 200  C, which is attributed to the transformation of active Cu2þ species to less active [CueOeCu]2þ or clustering CuOx species (Verma et al., 2014). Interestingly, a seagull-like NO conversion curve with two maxima at 250  C and 425  C, respectively,

Fig. 6. NO conversion as a function of temperature for H- and Cu-SSZ-13 zeolites. The test with quartz wool (without any zeolite catalyst) was performed as a control experiment.

was noticed over low-loaded Cu-0.14-SSZ-13 catalyst, which is in good agreement with previous report (Gao et al., 2017). The lowtemperature maxima is known to result from the formation of NH3-solvated, highly mobile and reactive Cu(NH3)n species at low temperatures (Gao et al., 2017; Paolucci et al., 2016, 2017). 3.4. Catalytic relevance of NHþ 4 intermediates The catalytic relevance of NHþ 4 intermediates was investigated by examining the NH3-SCR efficiency of Cu-SSZ-13 zeolites after exposure to NO/NH3 mixture at 200  C. The activities of freshly activated Cu-SSZ-13 zeolites were also examined as reference. As shown in Fig. 7, NO conversion of the NO/NH3-treated Cu-SSZ-13 (Cycle 2) was clearly higher than that of the freshly activated

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financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Valentina Rizzotto: Data curation, Writing - review & editing. € rn Dongdong Chen: Writing - original draft, Formal analysis. Bjo Martin Tabak: Investigation, Methodology. Jia-Yue Yang: Software. Daiqi Ye: Writing - review & editing. Ulrich Simon: Conceptualization, Writing - review & editing. Peirong Chen: Conceptualization, Writing - review & editing, Supervision. Acknowledgments

Fig. 7. NO conversions at 200  C for Cu-SSZ-13 zeolite catalysts in freshly activated state (cycle 1) and after pre-treatment in NO/NH3 (cycle 2).

catalyst (Cycle 1). Such enhancement in NO conversion follows the order Cu-0.18-SSZ-13 > Cu-0.24-SSZ-13 > Cu-0.14-SSZ-13, which is closely related to the relative content of the in situ generated NHþ 4 species resulted from the Cu2þ reduction by NO/NH3 (Figs. S4e5). As revealed in our previous study (Chen et al., 2018), the NHþ 4 intermediates tend to be closely coupled with Cuþ forming Cuþ/NHþ 4 species, which can react with NO/O2 to produce Cu2þNO2/NHþ 4, a key intermediate for the rate-limiting Cuþ / Cu2þ re-oxidation in NH3-SCR over Cu-CHA (Paolucci et al., 2014, 2016, 2017; Gao et al., 2017). Based on the in situ DRIFTS and catalytic studies, we propose that the NHþ 4 intermediates are involved in NH3-SCR catalysis by mediating the Cu redox cycle, in particular the Cuþ / Cu2þ reoxidation. It is worth noting that, at the selected reaction temperature (i.e. 200  C), H2O molecules can be hardly adsorbed on Cu sites even with a significantly higher concentration than that of NH3 (Paolucci et al., 2016). Therefore, we believe the findings in this work are also valid in common NH3-SCR catalysis with H2O in the reaction mixture. 4. Conclusions In summary, a series of Cu-SSZ-13 catalysts with different Cu ion exchange levels were prepared to understand the formation and catalytic relevance of NHþ 4 intermediates during NH3-SCR. NH3-TPD and DR UV/Vis analyses revealed that Cu species in Cu-SSZ-13 were present as isolated Cu2þ sites at low loadings (Cu/Al ratio lower than 0.18), and were transformed partially to [CueOeCu]2þ dimers or very small CuOx clusters at a high Cu loading (Cu/Al ¼ 0.24). In situ DRIFTS identified the formation of NHþ 4 intermediates resulting from the reduction of Cu2þ by co-adsorbed NH3 and NO. In NH3SCR, the highest catalytic performance was observed over Cu-0.18SSZ-13, whereas a further increase of Cu loading was detrimental for the reaction because of a loss of active Cu2þ sites. Further catalytic studies over NO/NH3-treated Cu-SSZ-13 catalysts unveiled a close correlation between the NO conversion and the amount of in  situ formed NHþ 4 intermediates at 200 C, which is attributed to a þ mediating effect of NH4 intermediate on the Cu redox cycle, in particular the Cuþ / Cu2þ re-oxidation. Our study confirms that þ NHþ 4 intermediates play a vital role in Cu re-oxidation and can promote the reactivity of Cu-SSZ-13 in NH3-SCR reactions. Declaration of competing interest The authors declare that they have no known competing

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