Excellent Activity and Selectivity of One-Pot Synthesized CuâSSZ-13

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Excellent activity and selectivity of one-pot synthesized Cu-SSZ-13 catalyst in the selective catalytic oxidation of ammonia to nitrogen Tao Zhang, Huazhen Chang, Yanchen You, Chuanning Shi, and Junhua Li Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00267 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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Environmental Science & Technology

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Excellent activity and selectivity of one-pot synthesized Cu-SSZ-13 catalyst

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in the selective catalytic oxidation of ammonia to nitrogen

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Tao Zhang, Huazhen Chang,*, Yanchen You, Chuanning Shi, Junhua Li

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†

†

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†

†

‡

School of Environment & Natural Resources, Renmin University of China, Beijing 100872, China

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†

‡

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, 100084, China

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* Corresponding authors: E-mail address: [email protected]

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Tel.: +86-10-62512572

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Abstract

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One-pot synthesized Cu-SSZ-13 catalyst treated with dilute HNO3 achieved superior

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activity and selectivity in the selective catalytic oxidation (SCO) of NH3 to nitrogen, in

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comparison with other zeolite-based catalysts and most of metal-oxide catalysts. Furthermore,

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the catalyst showed the similar or even higher catalytic activity than the partial noble-metal

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catalysts, and meanwhile its N2 selectivity was superior to most noble-metal catalysts. The

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characterization results demonstrated that more Cu2+ ions existing in Cu-SSZ-13 catalyst were

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advantageous to its NH3-SCO activity. The in situ DRIFTS results indicated that the reactivity

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of NH3 species adsorbed on Lewis and Brønsted acid sites over Cu-SSZ-13-O-H catalyst

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depended on the reaction temperature. The results of this study suggest the one-pot

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synthesized Cu-SSZ-13 is a promising NH3-SCO catalyst for practical application, either

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mobile or stationary pollution sources.

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1. Introduction

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The selective catalytic reduction (SCR) of NOx with NH3 is one of the most promising

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technologies for the NOx removal in mobile and stationary pollution sources.1,2 In order to

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achieve the desired NOx conversions, a stoichiometric or even an excess quantity of NH3 is

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required. However, this condition could result in unreacted NH3, which is called NH3 slip.3 In

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this case, an oxidation catalyst is usually employed to selectively oxidize the unreacted NH3

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from SCR units into nitrogen and water vapour (i.e., NH3-SCO reaction).

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Different types of catalysts have been reported to be active for the NH3-SCO reaction. They

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could be probably classified into three groups: (1) nobile metal catalysts;4,5 (2) metal oxide

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catalysts;6,7 (3) zeolite catalysts.8,9 Nobile metal catalysts show excellent low-temperature

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SCO activity, whereas high selectivity towards NOx is simultaneously found for these

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catalysts due to deeper oxidation of NH3. Another obvious drawback for nobile metal 2 ACS Paragon Plus Environment

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catalysts is a relatively high cost, which makes them less attractive for practical application.

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Metal oxide and zeolite catalysts are the other promising catalysts studied widely in the

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literatures. These two types of catalysts show higher N2 selectivity, however, need higher

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operation temperatures, than nobile metal catalysts.10 Nevertheless, Cu-based catalysts are

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one of the most efficient catalyst systems in the NH3-SCO reaction and have attracted more

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and more attentions.11

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Since zeolites possess the uniform strength and location of surface acid sites and

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ion-exchange properties, Cu-exchanged zeolite catalysts are an alternative class of NH3-SCO

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catalysts,11 including Cu/ZSM-5, Cu/Y, Cu/Beta, and so on. Among them, small pore

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Cu-exchanged SSZ-13 has been commercialized as SCR catalysts in diesel engines due to its

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much improved activity, selectivity, and hydrothermal stability.12 In recent years, some

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alternative Cu-SSZ-13 synthesis methods have been developed, such as aqueous

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ion-exchange method,12 solid-state ion-exchange method,13 and one-pot synthesis method.14,15

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Compared with the other two methods, one-pot synthesis method could introduce more Cu2+

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ions into ion-exchange sites of SSZ-13 support, because its small pore openings makes the

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introduction of Cu2+ ions more difficult by ion-exhange methods. Therefore, one-pot

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Cu-SSZ-13 catalyst exhibited more excellent NH3-SCR activity.16 In addition, Gao et al.

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employed Cu-SSZ-13 catalysts with varying Cu loadings and Si/Al ratios to examine their

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catalytic properties using NO oxidation, NH3 oxidation, standard NH-SCR reactions.17,18

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Herein, Cu-SSZ-13 catalyst was first prepared by ono-pot synthesis method. Then, this

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sample was treated with dilute HNO3 solution. The two catalysts before and after acid

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treatment were applied into the NH3-SCO reaction to test their catalytic performances. BET,

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XRD, HRTEM, EPR, XPS, H2-TPR ,NH3-TPD, and in situ DRIFTS were used to characterize

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the structure, types of Cu species, redox and acidic properties for the two catalysts. Finally,

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active Cu species and reaction mechanism were discussed. 3 ACS Paragon Plus Environment


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2. Experimental

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2.1. Preparation of the catalysts.

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The pristine Cu-SSZ-13 catalyst was prepared using one-pot synthesis method as reported

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previously and denoted as Cu-SSZ-13-O.14 To adjust Cu loadings, the above sample before

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calcining was treated with dilute HNO3 solution (PH = 1) at 80 °C for 4 h. After being filtered

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and washed with deionized water, the sample was dried overnight and calcined at 550 °C for 8

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h. The final sample was denoted as Cu-SSZ-13-O-H. In addition, the Cu loadings and

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ion-exchange levels for these two samples were determined by ICP analysis and the results

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are shown in Table 1.

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Furthermore, H-SSZ-13 and CuO/Cu-SSZ-13-O-H were prepared for comparison. The

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Cu-SSZ-13-O-H sample was ion-exchanged using a dilute HNO3 solution (PH = 1) at 80 °C

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for 4 h. Then, the suspension was filtered and washed with deionized water. The obtained

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sample was dried at 100 °C overnight before repeating this ion exchange process two times.

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The final product was calcined at 550 °C for 8 h and denoted as H-SSZ-13. ICP results

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showed that no Cu species were detected in this sample. The preparation procedure of CuO/

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Cu-SSZ-13-O-H was described as below. Quantitative Cu(NO3)2 (0.0775 g) was dissolved in

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0.6 mL deionized water. Next, 1.0 g Cu-SSZ-13-O-H was slowly added to the aqueous

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solution with thoroughly stirring. Then, the mixture was dried at 100 °C overnight and

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calcined at 550 °C for 6 h.

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Conventional V2O5-WO3/TiO2 catalyst with 1 wt.% V2O5 and 5 wt.% WO3 was prepared

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by an incipient wetness method using NH4VO3 and (NH4)10W12O41 as precursors,

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H2C2O4·2H2O as cosolvent, and P25 as support.

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2.2. Characterization of the catalysts. The elemental compositions of the samples were determined by ICP (PE, OPTIMA 4 ACS Paragon Plus Environment

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5300DV). Power X-ray diffraction (XRD) measurements were conducted on a Rigaku

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D/max-2200 diffractometer with Cu Kα radiation at a rate 10°/min over a 2θ range of 5-40°.

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High resolution transmission electron microscopy (HRTEM) images were obtained using a

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JEOL JEM 2100 electron microscope at an accelerating voltage of 200 kV. N2 adsorption/

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desorption analysis was carried out at 77 K using a Quantachrome Autosorb-6 analyzer.

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Electron paramagnetic resonance (EPR) measurements were performed at 155 K on a JEOL

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X-band spectrometer. X-ray photoelectron spectroscopy (XPS) was measured on an

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ESCALab220i-XL electron spectrometer with 300 W Mg Kα radiation. H2 temperature

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programmed reduction (H2-TPR) was performed on a ChemiSorb 2720 TPx chemisorption

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analyzer. Typically, 0.1 g sample was pretreated in Ar at 300 °C for 1 h. After that, H2-TPR

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was conducted in 10% H2/Ar at a flow rate of 50 mL/min and the temperature was ramped

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from ambient to 700 °C at 10 °C/min. NH3 temperature programmed desorption (NH3-TPD)

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was carried out using an ammonia analyzer (LGR, EAA-30r-EP). After pretreating the

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samples in pure N2 at 500 °C for 1 h, the chemisorption of NH3 was conducted at 100 °C

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untile adsorption equilibrium was reached. Finally, the samples were heated from 100 to

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700 °C at 10 °C/min. In situ DRIFTS spectra were recorded on a Nicolet NEXUS 6700

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spectrometer. Before the experiment, the samples were pretreated at in pure N2 at 500 °C for 1

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h. The IR spectra were collected by accumulating 32 scans at a spectral resolution of 4 cm-1.

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2.3. Catalyst evaluation.

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The NH3-SCO performances of the catalysts (40-60 mesh) were tested in a fixed-bed

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quartz flow reactor. The feed gas contained 500 ppm NH3, 5% O2, 5% H2O (when used)

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and banlance N2, with a gas hourly space velocity (GHSV) of ~160 000 h-1. The

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concentrations of reactants and products were monitored by an online FTIR spectrometer

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(MultiGas 2030HS, MKS). The NH3 conversion and N2 selectivity were calculated using 5 ACS Paragon Plus Environment


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following equations:

NH 3 Conversion=

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[NH 3 ]inlet − [NH 3 ]outlet × 100% [NH 3 ]inlet

 [NO]outlet +[NO 2 ]outlet +2[N 2 O]outlet N 2 Selectivity =  1 − [NH 3 ]inlet − [NH 3 ]outlet 

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3. Results

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3.1. Catalytic activity

(1)

  × 100% 

(2)

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The catalytic performances of Cu-SSZ-13-O and Cu-SSZ-13-O-H catalysts were evaluated

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and the results are shown in Figure 1A. For comparison, the catalytic performances of the

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H-SSZ-13 and conventional V2O5-WO3/TiO2 catalysts (VWTi) were also tested. H-SSZ-13

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and VWTi showed poor NH3-SCO activity in the whole temperature range and the N2

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selectivity decreased obviously at temperature above 350 °C (Figure 1B). On the contrary,

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Cu-SSZ-13-O and Cu-SSZ-13-O-H exhibited excellent NH3-SCO activity and NH3

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conversion was above 90% when the reaction temperature increased at 200 °C and above.

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Furthermore, the N2 selectivity remained above 90% in the whole temperature range over

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these two catalysts (Figure 1B). Therefore, both the catalytic activity and N2 selectivity could

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be simultaneously enhanced after the addition of Cu onto HSSZ-13. Note that

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Cu-SSZ-13-O-H showed slightly higher NH3 conversion than Cu-SSZ-13-O at low

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temperatures (≤ 200 °C) as well as N2 selectivity (Figure 1B). And this difference in the

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NH3-SCO activities of the two catalysts was even more distinct under a higher GHSV (Figure

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S1A).

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Furthermore, the NH3-SCO performances of Cu-SSZ-13-O and Cu-SSZ-13-O-H catalysts

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in the presence of H2O were also tested. As shown in Figure 1A, the presence of 5% H2O

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resulted in a fall in the NH3 conversion at low temperatures (≤ 300 °C), which might be due to

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the competitive adsorption of H2O and NH3. Nevertheless, the N2 selectivities of thses two

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catalysts were almost unaffected by the introduction of H2O. 6 ACS Paragon Plus Environment

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In order to compare the catalytic performance of Cu-SSZ-13-O-H catalyst, the NH3-SCO

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activities of the reported catalysts in literatures were summarized in Table S1. It is important

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to note that the reaction conditions were generally different from each other, including the

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feed-gas composition, total flow, catalyst doseage, even measurement system. Therefore, it is

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hard to compare the catalytic performances of various catalysts in detail. However, a rough

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comparison is still feasible. As shown in Table S1, Cu-SSZ-13-O-H presented more excellent

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catalytic performance across the entire test temperature compared with all of other

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zeolite-based catalysts and most of metal-oxide catalysts except the CuO/carbon nano-tubes

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and MnOx. Moreover, Cu-SSZ-13-O-H showed the similar or even higher catalytic activity

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than the partial noble metal catalysts, and meanwhile its N2 selectivity was superior to most

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noble-metal catalysts. For noble-metal catalysts, deep oxidation of NH3 to nitrogen oxides

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easily occurs due to their excellent redox properties.10 From the above, Cu-SSZ-13-O-H is a

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promising catalyst for practical application in NH3-SCO process due to its high catalytic

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activity and N2 selectivity.

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3.2. Catalyst characterization

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3.2.1. Structure of the catalysts studied by XRD, HRTEM, BET, and EPR

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Figure 2A shows the XRD patterns of Cu-SSZ-13-O and Cu-SSZ-13-O-H catalysts. These

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two catalysts exhibited the typical patterns of the CHA zeolite structure.19 The crystalline

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phase ascribed to CuO species (2θ = 35.6° and 38.8°) was observed over Cu-SSZ-13-O

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catalyst, suggesting that the partial Cu species existed as CuO phase on its external surface. In

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contrast, no diffraction peaks of CuO species were observed over Cu-SSZ-13-O-H catalyst,

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indicating that Cu species were dispersed very well on its surface. Importantly, it was noted

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that the intensity of the diffraction peak at ~9.5° of Cu-SSZ-13-O-H was slightly lower than

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that of Cu-SSZ-13-O. This peak is generated from the (100) diffraction patterns reflections of 7 ACS Paragon Plus Environment


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the CHA crystal and easily weakened by Cu2+ ions that cause the deformation of the

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8-membered rings of the CHA cages.20 Therefore, lower peak intensity manifests that more

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isolated Cu2+ ions close to the 8-membered rings exist in corresponding sample.

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Measured HRTEM images of Cu-SSZ-13-O and Cu-SSZ-13-O-H catalysts are shown in

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Figure 2B and 2C. Obviously, a large number of CuO nanoparticles were observed over

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Cu-SSZ-13-O. The mean diameters of these nanoparticles were by statistic analysis and the

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average particle size was 3.55 nm (illustration in Figure 2B). After post-treatment with dilute

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HNO3, CuO nanoparticles were difficult to be distinguished on the external surface of

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Cu-SSZ-13-O-H. The above phenomena are well consistent with the XRD results.

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The BET areas (SBET) and micro-pore volumes (Vmic) were derived from N2 physisorption

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and the results are shown in Table 1. Compared with Cu-SSZ-13-O, the SBET and Vmic of

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Cu-SSZ-13-O-H increased from 287.6 to 440.2 m2 g-1 and 0.13 to 0.21 cm3 g-1. This might be

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due to the absence of CuO nanoparticles, which could clog the micropores of SSZ-13 support.

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The coordination of Cu2+ ions in Cu-SSZ-13-O and Cu-SSZ-13-O-H catalysts could be

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investigated by EPR analysis, because CuO and Cu+ ions do not show any EPR signals.21 As

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shown in Figure S2, α (g|| = 2.37 and A|| = 136 G) and β (g|| = 2.34 and A|| = 149 G) denoted

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Cu2+ ions close to the 6- and 8-membered rings, respectively.22 This indicates that there are

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two types of Cu ions existing in the two catalysts at different ion exchange positions. Further,

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the peak width of Cu-SSZ-13-O-H was wider than that of Cu-SSZ-13-O, suggesting that more

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Cu2+ ions presented in the former.23

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3.2.2. Redox properties

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The O 1s XPS spectra of Cu-SSZ-13-O and Cu-SSZ-13-O-H catalysts are shown in Figure

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S3A. Each spectum could be fitted into two peaks, assigned to the lattice oxygen (Olatt) at

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530.8 eV and the the adsorbed oxygen (Oads) at 531.9 eV.24 As is known to all, Oads is more 8 ACS Paragon Plus Environment

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active in oxidation reactions than Olatt owing to its higher mobility. The ratio of Oads/ Olatt for

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Cu-SSZ-13-O-H (5.39) was higher than that for Cu-SSZ-13-O (3.09), suggesting that the

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surface active oxygen species could be improved by post-treatment with dilute HNO3.

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Furthermore, the Cu 2p XPS spectra indicated that Cu species on the surface of these two

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catalyst existed as Cu2+ (Figure S3B).

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The reducibilities of Cu-SSZ-13-O and Cu-SSZ-13-O-H catalysts were confirmed by the

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H2-TPR experiments and the results are shown in Figure 3A. Furthermore, the peak fitting

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process was performed to make H2 reduction peaks below 500 °C more accurate. Based on

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the previous studies,25,26 the reduction peaks at 237-248 °C were attributed to the reduction of

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Cu2+ close to 8-membered rings to Cu+, while the reduction peaks at 324-336 °C were

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ascribed to the reduction of Cu2+ close to 6-membered rings to Cu+. Besides, according to the

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H2-TPR results of nanosized CuO (Figure 3A), the reduction peak at 299 °C only appearing

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on Cu-SSZ-13-O was due to the reduction of CuO to Cu0. Note that the position of the first

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reduction peak for Cu-SSZ-13-O-H (237 °C) was slightly lower than that for Cu-SSZ-13-O

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(248 °C), indicating that Cu-SSZ-13-O-H owned a better low-temperature reducibility.

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Moreover, the integral areas of the reduction peaks at 237-248 and 324-336 °C as a function

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of the catalysts are depicted in Figure 3B. The peak areas of two Cu2+ species for

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Cu-SSZ-13-O-H were larger than those for Cu-SSZ-13-O, suggesting that Cu-SSZ-13-O-H

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possessed more Cu2+ ions both close to 8- and 6-membered rings. In addition, the reduction

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peaks above 500 °C were assigned to the reduction of Cu+ to Cu0.26

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3.2.3. Surface acidities

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The acidities of Cu-SSZ-13-O and Cu-SSZ-13-O-H catalysts were determined by

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NH3-TPD experiments and the results are shown in Figure 4A. There were three NH3

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desorption peaks (labeled as α, β, γ) observed over the two catalysts. According to analyses in 9 ACS Paragon Plus Environment


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the previous studies,27,28 peak α was attributed to weakly adsorbed NH3, for instance, NH3

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adsorbed on weak Lewis acid sites and physisorbed NH3; peak β was assigned to NH3

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adsorbed on strong acid sites, mainly from Cu2+ ions in ion-exchanged sites; peak γ was

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ascribed to NH3 adsorbed on Brønsted acid sites, created by ZCuOH (Cu2+ ions coordinated

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with Zeolite) and ZAlOH (framework Al atoms). Furthermore, the three NH3-adsorbed

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species could be quantified by peak deconvolution. As shown in Figure 4A and 4B, the

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amount of each NH3-adsorbed species for Cu-SSZ-13-O-H was larger than that for

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Cu-SSZ-13-O, especially NH3 adsorbed on Lewis and Brønsted acid sites (peak β and γ). This

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might be due to the following: (1) the EPR and H2-TPR results showed that more Cu2+ ions

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exsited in Cu-SSZ-13-O-H, which therefore owned more abundant Lewis acid sites; (2)

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partial Brønsted acid sites of Cu-SSZ-13-O were covered by CuO nanoparticles on its

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external surface, which was inimical to NH3 adsorption.

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3.3. In situ DRIFTS studies

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3.3.1 NH3 adsorption

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Figure S4A shows the DRIFT spectra of NH3 adsorption on Cu-SSZ-13-O-H catalyst at

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100 °C followed by purge in N2 at different temperatures. Several bands at 1458, 1619, 3178,

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3267, 3362, 3591, 3667 and 3730 cm-1 were observed. The band at 1458 cm-1 was due to

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NH4+ species adsorbed on Brønsted acid sites, while the band at 1619 cm-1 was related to

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coordinated NH3 bound to Lewis acid sites.29-31 Three bands in the range of 3100 to 3400

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cm-1 were assigned to N-H stretching vibrations. Specially, the bands at 3267 and 3362 cm-1

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were attributed to NH4+ group.32 The band at 3178 cm-1 was assigned to NH3 adsorbed on

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Cu2+ sites on SSZ-13,33,34 which could be further verified by Figure S5. Furthermore, the

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negative bands at 3591, 3606, 3667 and 3730 cm-1 were attributed to OH vibrations.30,35

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Note that the stretching OH band at 3606 cm-1 was assigned to the v(OH) stretch of 10 ACS Paragon Plus Environment

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[Cu2+(OH)-]+ species,28,36,37 and the other three bands were attributed to the depletion of

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Si-OH-Al acid sites by NH3.33 The intensities of all the bands were weakened as the

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temperature increased, apparently above 200 °C.

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The ratios of Lewis and Brønsted acidities (L/B) were obtained by calculating the

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integrated peak areas of 1619 and 1458 cm-1 in the DRIFTS spectra at different temperatures,

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and the results are shown in Figure 5. When the temperature increased, the L/B ratios

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followed a monotonic decreasing trend with increasing temperature. This result suggests that

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NH4+ species adsorbed on Brønsted acid sites were more stable than the coordinated NH3 on

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Lewis acid sites.

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3.3.2. Interaction of NH3 with O2

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The machanism of NH3 oxidation was investigated via the behavior of NH3-adsorbed

248

species interacting with O2 over Cu-SSZ-13-O-H catalyst using in situ DRIFTS. Figure S4B

249

shows the DRIFT spectra of NH3 adsorption on the catalyst at 100 °C followed by purge in

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5%O2/N2 at different temperatures. The bands of NH3 ascribed to Lewis acid sites (1619 cm-1)

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and Brønsted acid sites (1458 cm-1) decreased gradually with increasing temperature. The L/B

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ratios were also calculated and the results are shown in Figure 5. The L/B ratios decreased

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monotonously below 200 °C. However, the L/B ratios increased between 200 and 300 °C and

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then decreased above 300 °C.

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In addition, compared with the sample purged only in N2, the L/B ratios for the sample

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purged in 5%O2/N2 were smaller below 200 °C. This suggests that the reaction between O2

257

and the coordinated NH3 on Lewis acid sites takes place at a faster rate than that for the NH4+

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species on Brønsted acid sites, and therefore the coordinated NH3 on Lewis acid sites was

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more active than the NH4+ species on Brønsted acid sites in this temperature range.

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Nevertheless, it was found that the L/B ratios for the sample purged in 5%O2/N2 became 11 ACS Paragon Plus Environment


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larger than those for the sample purged in N2 above 200 °C. The above observations clearly

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indicate that the reactivity of NH3 species adsorbed on Lewis and Brønsted acid sites over

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Cu-SSZ-13-O-H is correlated with the reaction temperature.

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4. Discussion

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According to the ICP results shown in Table 1, Cu-SSZ-13-O-H had a very similar Si/Al

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ratio with Cu-SSZ-13-O, indicating that no apparent dealumination occurred during the

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process of post-treatment with dilute HNO3. In contrast, the Cu loadings in Cu-SSZ-13-O was

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obviously higher than that in Cu-SSZ-13-O-H as well as ion-exchange level. For

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Cu-SSZ-13-O, the ion-exchange level was 111.2%. In principle, there are enough Cu2+ ions to

271

balance the framework charges of SSZ-13 support. However, the EPR, H2-TPR and NH3-TPD

272

results showed that more abundant Cu2+ ions exsited in Cu-SSZ-13-O-H in comparison with

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Cu-SSZ-13-O. As displayed in Figure 2B, a large number of CuO nanoparticles were

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observed on the external surface of Cu-SSZ-13-O. Thus, post-treatment with dilute HNO3

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might have three positive effects: (1) eliminating excess Cu species in the initial Cu-SSZ-13

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product; (2) boosting the quantity of Cu2+ ions; (3) optimizing the spatial distribution of Cu2+

277

ions in SSZ-13 stucture.

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From the results of activity test shown in Figure 1A, HSSZ-13 showed almost no activity in

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NH3 oxidition, meaning that NH3-SCO reaction mainly occurred on Cu sites. Further, it seems

280

that the isolated Cu2+ ions are the catalytically active centers, because the activity of

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Cu-SSZ-13-O-H was better than that of Cu-SSZ-13-O and Cu species in Cu-SSZ-13-O-H

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were in the form of Cu2+ ions. In addition, this hypothesis can be further verified by NH3

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TOFs calculated for Cu-SSZ-13-O-H and Cu/Cu-SSZ-13-O-H catalysts. For standard

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NH3-SCR reaction over the Cu-SSZ-13 catalyst, a general agreement about active centers is

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still lacking. Many studies reported that Cu2+ and [CuII(OH)]+ were the active centers.38-40 12 ACS Paragon Plus Environment

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Gao ea al. thought that the low-temperature SCR reaction was achieved on a

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[CuI(NH3)2]+-O2-[CuI(NH3)2]+ intermediate (formed from two isolated CuI ions),41 while

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isolated Cu ions acted as individual active sites at high temperatures. Paolucci et al. revealed

289

that transient ion pairs [(NH3)2CuII]-O2-[(NH3)2CuII] (formed from mobilized Cu ions) acted

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as the active centers by participating in an O2-mediated CuI → CuII redox step.42 For

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NH3-SCO reaction over the Cu-SSZ-13 catalyst, reaction kinetics studies suggested that

292

Cu-ion dimers were the actual active centers at low temperatures. In contrast, Cu-ion

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monomers became the active centers at the reaction temperature of 350 °C and above.18

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However, the existence of Cu-ion dimers has not been directly confirmed by the spectroscopic

295

method and the results of reaction kinetics are only indirect evidence for their existence. In

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any case, the active centers have a close relationship with Cu2+ ions over the Cu-SSZ-13

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catalyst, both for NH3-SCR reaction and for NH3-SCO reaction. Meanwhile, more abundant

298

Cu2+ ions existing in the Cu-SSZ-13 catalyst are beneficial for improving its NH3-SCO

299

activity, which is due to that more gasous NH3 could be adsorbed on Lewis acid sites (mainly

300

from Cu2+ ions) to participate in the reaction. The DRIFTS results shown in Figure S4B

301

indicated that coordinated NH3 bound to Lewis acid sites was more active than NH4+ species

302

adsorbed on Brønsted acid sites at 200 °C and below. This might explain the results shown in

303

Figure 1A: the activity of Cu-SSZ-13-O-H was better than that of Cu-SSZ-13-O at low

304

temperatures.

305

Two possible NH3-SCO mechanisms have been proposed, i.e., a N2H4 route43 and a

306

two-step SCR-like route9 As shown in Figure S4B, the DRIFTS results showed that the peak

307

at ~1606 cm-1 assigned to N2H4 species9 was not observed in the presence of O2, indicating

308

that the N2H4 route was unfit for Cu-SSZ-13-O-H catalyst. Furthermore, only adsorbed NH3

309

species desorbed or reacted under 5%O2/N2 purging and no NOx species were observed

310

adsorbing on the catalyst surface. According to the catalytic performance and the results of O 13 ACS Paragon Plus Environment


Page 14 of 29

311

1s XPS and H2-TPR, post-treatment with dilute HNO3 for one-pot synthesized Cu-SSZ-13-O

312

catalyst was beneficial to oxygen adsorption and activation, the low-temperature reducibility

313

of the catalyst was thus improved. Therefore, a high activity for NH3-SCO is expected. On the

314

base of above analyses, the NH3-SCO reaction on Cu-SSZ-13-O-H catalyst might take place

315

by the two-step SCR-like route: NH3 is first oxidized to NO by surface active oxygen species,

316

which occurs on the catalyst surface. Subsequently, the NO reacts with surplus NH3 and is

317

reduced to N2 (NH3-SCR reaction). Note that more Cu2+ ions existing in the Cu-SSZ-13

318

catalyst were favourable for the second-step reaction at low temperatures.23 This could be one

319

reason that explains the excellent low-temperature NH3-SCO activity of Cu-SSZ-13-O-H.

320 321 322 323

Supporting Information Review of NH3-SCO performances of different catalysts, NH3-SCO performances of

324

Cu-SSZ-13-O

and

Cu-SSZ-13-O-H

catalysts

under

different

GHSVs,

additional

325

characterization data of Cu-SSZ-13-O and Cu-SSZ-13-O-H catalysts including EPR spectra,

326

XPS spectra, and DRIFTS spectra, NH3-SCO performances and NH3 TOFs of

327

Cu-SSZ-13-O-H and CuO/Cu-SSZ-13-O-H catalysts.

328 329

Acknowledgements

330

This work was financially supported by the National Key R&D Program of China

331

(2016YFC0203900 and 2016YFC0203901), the Fundamental Research Funds for the Central

332

Universities, and the Research Funds of Renmin University of China (18XNLG09) and the

333

National Natural Science Foundation of China (Grants 21577173 and 51778619).

334 335

References 14 ACS Paragon Plus Environment

Page 15 of 29


336

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selective catalytic reduction of NOx by ammonia over oxide catalysts: A review. Appl.

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Catal. B 1998, 18 (1-2), 1-36.

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[2] Forzatti, P. Present status and perspectives in de-NOx SCR catalysis. Appl. Catal. A 2001, 222 (1-2), 221-236.

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[3] Chen, W.; Ma, Y.; Qu, Z.; Liu, Q.; Huang, W.; Hu, X.; Yan N. Mechanism of the Selective

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Catalytic Oxidation of Slip Ammonia over Ru-Modified Ce-Zr Complexes Determined by

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in Situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy. Environ. Sci.

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Technol. 2014, 48 (20), 12199-12205.

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[4] Gang, L.; Anderson, B. G.; van Grondelle J.; van Santen R. A. NH3 oxidation to nitrogen

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[5] Jablonska, M.; Krol, A.; Kukulska-Zajac, E.; Tarach, K.; Chmielarz, L.; Gora-Marek, K.

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Zeolite Y modified with palladium as effective catalyst for selective catalytic oxidation of

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ammonia to nitrogen. J. Catal. 2014, 316, 36-46.

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[6] Chmielarz, L.; Kustrowski, P.; Rafalska-Lasocha, A.; Dziembaj, R. Selective oxidation of

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ammonia to nitrogen on transition metal containing mixed metal oxides. Appl. Catal. B

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[7] Long, R. Q.; Yang, R. T. Selective catalytic oxidation of ammonia to nitrogen over Fe2O3-TiO2 prepared with a sol-gel method. J. Catal. 2002, 207 (2), 158-165. [8] Long, R. Q.; Yang, R. T. Superior ion-exchanged ZSM-5 catalysts for selective catalytic oxidation of ammonia to nitrogen. Chem. Commun. 2000, 0, 1651-1652.

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into nitrogen and water vapour-Recent trends and open challenges. Appl. Catal. B 2016,

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ion-exchange method. ChemCatChem 2014, 6 (6), 1579-1583.

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the roles of Brønsted acidity. J. Catal. 2015, 331, 25-38.

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evaluation of Cu/SAPO-34 catalysts for NH3-SCR 2: Solid-state ion exchange and

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one-pot synthesis. Appl. Catal. B 2015, 162, 501-514.

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[21] Xue, J.; Wang, X.; Qi, G.; Wang, J.; Shen M.; Li, W. Characterization of copper species

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over Cu/SAPO-34 in selective catalytic reduction of NOx with ammonia: Relationships

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between active Cu sites and de-NOx performance at low temperature. J. Catal. 2013, 297,

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56-64.

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[23] Zhang, T.; Qiu, F.; Chang, H.; Li, X.; Li. J. Identification of active sites and reaction

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mechanism on low-temperature SCR activity over Cu-SSZ-13 catalysts prepared by

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different methods. Catal. Sci. Technol. 2016, 6, 6294-6304

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[24] Zhang, Y.; Zhang, H.; Yan, Y. Metal-organic chemical vapor deposition of Cu(acac)2 for

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the synthesis of Cu/ZSM-5 catalysts for the oxidation of toluene. Micropor. Mesopor.

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Mat. 2018, 261, 244-251.

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reaction kinetics and EPR studies. J. Catal. 2013, 300, 20-29.

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insights into Cu/SSZ-13 SCR catalyst acidity. Part I: Nature of acidic sites probed by

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NH3 titration. J. Catal. 2017, 348, 291-299.

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Waroquier, M.; van Speybroeck, V.; Weckhuysen, B. M.; Beale, A. M. Determining the

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storage, availability and reactivity of NH3 within Cu-Chabazite-based ammonia selective

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catalytic reduction systems. Phys. Chem. Chem. Phys. 2014, 16 (4), 1639-1650.

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[29] Ma, L.; Cheng, Y.; Cavataio, G.; McCabe, R. W.; Fu, L.; Li, J. In situ DRIFTS and

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temperature-programmed technology study on NH3-SCR of NOx over Cu-SSZ-13 and

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Cu-SAPO-34 catalysts. Appl. Catal. B 2014, 156-157, 428-437.

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[30] Xie, L.; Liu, F.; Liu, K.; Shi X.; He, H. Inhibitory effect of NO2 on the selective catalytic

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reduction of NOx with NH3 over one-pot-synthesized Cu–SSZ-13 catalyst. Catal. Sci.

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Technol. 2014, 4, 1104-1110.

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[31] Chen, L.; Li, J.; Ge, M. DRIFT study on cerium-tungsten/titiania Catalyst for selective catalytic reduction of NOx with NH3. Environ. Sci. Technol. 2010, 44 (24), 9590-9596.

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Mussmann, L. Influence of NO2 on the selective catalytic reduction of NO with

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ammonia over Fe-ZSM5. Appl. Catal. B 2006, 67, 187-196.

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[33] Wang, L.; Li, W.; Qi, G. S.; Weng, D. Location and nature of Cu species in Cu/SAPO-34 for selective catalytic reduction of NO with NH3. J. Catal. 2012, 289, 21-29.

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[34] Giordanino, F.; Borfecchia, E.; Lomachenko, K. A.; Lazzarini, A.; Agostini, G.; Gallo,

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E.; Soldatov, A. V.; Beato, P.; Bordiga, S.; Lamberti, C. Interaction of NH3 with

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Cu-SSZ-13 catalyst: A complementary FTIR, XANES, and XES Study. J. Phys. Chem.

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Lett. 2014, 5(9), 1552-1559.

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selective catalytic reduction of NOx by NH3 over Cu-exchanged SAPO-34. ACS Catal. 18 ACS Paragon Plus Environment

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Beato, P.; Bordiga, S.; Lamberti, C. Characterization of Cu-exchanged SSZ-13: A

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comparative FTIR, UV-Vis, and EPR study with Cu-ZSM-5 and Cu-β with similar Si/Al

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and Cu/Al ratios. Dalton Trans. 2013, 42 (35), 12741-12761.

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movement of Cu ions in a SSZ-13 zeolite during dehydration, reduction and adsorption:

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A combined in situ TP-XRD, XANES/DRIFTS study. J. Catal. 2014, 314, 83-93.

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[38] Kwak, J. H.; Lee, J. H.; Burton, S. D.; Lipton, A. S.; Peden, C. H. F.; Szanyi, J. A

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common intermediate for N2 formation in enzymes and zeolites: Side-On Cu-nitrosyl

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complexes. Angew. Chem. Int. Ed. 2013, 52 (38), 9985-9989.

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Delgass, W. N.; Ribeiro, F. H.; Schneider, W. F. Isolation of the copper redox steps in the

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standard selective catalytic reduction on Cu-SSZ-13. Angew. Chem. Int. Ed. 2014, 53

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(44), 11828-11833.

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[40] Paolucci, C.; Parekh, A. A.; Khurana, I.; Di Iorio, J. R.; Li, H.; Albarracin Caballero, J.

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Schneider, W. F. Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of

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[41] Gao, F.; Mei, D.; Wang, Y.; Szanyi, J.; Peden, C. H. F. Selective catalytic reduction over

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Schneider, W. F.; Gounder, R. Dynamic multinuclear sites formed by mobilized copper

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[43] Ramis, G.; Yi L.; Busca, G. Ammonia activation over catalysts for the selective catalytic

464

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465

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Page 20 of 29

Page 21 of 29


Captions for Tables

466

1.

Physicochemical

467

Table

468

CuO/Cu-SSZ-13-O-H samples.

properties

of

Cu-SSZ-13-O,

469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493


Cu-SSZ-13-O-H

and


Page 22 of 29

494 495 496 497 498 499 500 501 502 503

1.

Physicochemical

504

Table

505

CuO/Cu-SSZ-13-O-H samples. Samples

properties

of

Cu-SSZ-13-O,

and

SBET (m2·g-1)a Vmic (cm3·g-1)b SiO2/Al2O3c Cu (wt.%) c Ion-exchange level (%)

Cu-SSZ-13-O

287.6

0.13

7.52

8.41

111.2

Cu-SSZ-13-O-H

440.2

0.21

7.54

6.38

82.7

CuO/Cu-SSZ-13-O-H

386.5

0.18

7.54

8.37

108.5

506 507

a

Calculated by BET method

508

b

Calculated by t-plot method

c

Obtain from ICP technique

509

Cu-SSZ-13-O-H

510 511 512 513 514 515 516


Page 23 of 29


517

Figure captions

518

Figure 1. (A) NH3 conversion and (B) N2 selectivity over four samples. Reaction conditions:

519

[NH3] = 500 ppm, [O2] = 5%, [H2O] = 5% (when used), N2 balance, total flow rate 200 mL

520

min-1 and GHSV = 160,000 h-1.

521

Figure 2. (A) XRD patterns of (a) Cu-SSZ-13-O and (b) Cu-SSZ-13-O-H. HRTEM images of

522

(B) Cu-SSZ-13-O and (C) Cu-SSZ-13-O-H.

523

Figure 3. (A) H2-TPR profiles of (a) CuO, (b) Cu-SSZ-13-O and (c) Cu-SSZ-13-O-H. (B)

524

Correlation between amounts of relevant H2 consumption and the catalysts.

525

Figure 4. (A) NH3-TPD profiles of (a) Cu-SSZ-13-O and (b) Cu-SSZ-13-O-H. (B)

526

Correlation between amounts of relevant desorbed-NH3 species and the catalysts.

527

Figure 5. The ratios of Lewis and Brønsted acidities at different temperatures (obtained by

528

calculating the integrated peak areas of 1619 and 1458 cm-1 in Figure S4) upon passing N2 or

529

5%O2/N2 over Cu-SSZ-13-O-H with preadsorbed NH3.

530



Page 24 of 29

531 532 533 534 535 536 537 538 100

(A)

80

80 HSSZ-13 Cu-SSZ-13-O Cu-SSZ-13-O-H VWTi Cu-SSZ-13-O + 5% H2O

60

40

Cu-SSZ-13-O-H + 5% H2O

20

(B) 0

150 200 250 300 350 400 150 200 250 300 350 400

60

40

N2 Selectivity / %

NH3 Conversion / %

100

20

0

o

539

Temperature / C

540

Figure 1. (A) NH3 conversion and (B) N2 selectivity over four samples. Reaction conditions:

541

[NH3] = 500 ppm, [O2] = 5%, [H2O] = 5% (when used), N2 balance, total flow rate 200 mL

542

min-1 and GHSV = 160,000 h-1.

543 544 545 546 547 548 549 550 551


Page 25 of 29


552 553 554 555 556 557

558 559

Figure 2. (A) XRD patterns of (a) Cu-SSZ-13-O and (b) Cu-SSZ-13-O-H. HRTEM images of

560

(B) Cu-SSZ-13-O and (C) Cu-SSZ-13-O-H.

561 562 563 564 565 566



Page 26 of 29

567 568 569 570 571 572 573 574

(A)

(B)

237

237

248 613

c 248 299

336

598

b

336

300

a 100

×1/20

200

300

400

500

600

700 Cu-SSZ-13 Cu-SSZ-13-H

o

575

324

Peak Area / a.u.

Intensity / a.u.

324

Temperature / C

Sample

576

Figure 3. (A) H2-TPR profiles of (a) CuO, (b) Cu-SSZ-13-O and (c) Cu-SSZ-13-O-H. (B)

577

Correlation between amounts of relevant H2 consumption and the catalysts.

578 579 580 581 582 583 584 585


Page 27 of 29


586 587 588 589 590 591 592 593 β

(B)

(A)

γ

α

α

β

Peak Area / a.u.

NH3 Concentration / ppm

50 ppm

γ

b

a 100 200 300 400 500 600 700 o

594

a

b Sample

Temperature / C

595

Figure 4. (A) NH3-TPD profiles of (a) Cu-SSZ-13-O and (b) Cu-SSZ-13-O-H. (B)

596

Correlation between amounts of relevant desorbed-NH3 species and the catalysts.

597 598 599 600 601 602 603 604 605



Page 28 of 29

606 607 608 609 610 611 612 613

Ratio of L and B / a.u.

0.3

N2 purge 5% O2/N2 purge

0.2

0.1

0.0

100

150

200

250

300

350

400

o

614

Temperature / C

615

Figure 5. The ratios of Lewis and Brønsted acidities at different temperatures (obtained by

616

calculating the integrated peak areas of 1619 and 1458 cm-1 in Figure S4) upon passing N2 or

617

5%O2/N2 over Cu-SSZ-13-O-H with preadsorbed NH3.

618 619 620 621 622 623 624 625 626


Page 29 of 29


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Excellent Activity and Selectivity of One-Pot Synthesized CuâSSZ-13

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