On the synthesis of a hierarchically-structured ZSM-5 zeolite and the effect of its physicochemical properties with Cu impregnation on cold-start hydrocarbon trap performance

Accepted Manuscript Title: On the synthesis of a hierarchically-structured ZSM-5 zeolite and the effect of its physicochemical properties with Cu impregnation on cold-start hydrocarbon trap performance Authors: Heejoong Kim, Eunhee Jang, Yanghwan Jeong, Jinseong Kim, Chun Yong Kang, Chang Hwan Kim, Hionsuck Baik, Kwan-Young Lee, Jungkyu Choi PII: DOI: Reference:

S0920-5861(18)30059-2 https://doi.org/10.1016/j.cattod.2018.02.008 CATTOD 11233

To appear in:

Catalysis Today

Received date: Revised date: Accepted date:

15-10-2017 17-1-2018 2-2-2018

Please cite this article as: Heejoong Kim, Eunhee Jang, Yanghwan Jeong, Jinseong Kim, Chun Yong Kang, Chang Hwan Kim, Hionsuck Baik, Kwan-Young Lee, Jungkyu Choi, On the synthesis of a hierarchically-structured ZSM-5 zeolite and the effect of its physicochemical properties with Cu impregnation on cold-start hydrocarbon trap performance, Catalysis Today https://doi.org/10.1016/j.cattod.2018.02.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

On the synthesis of a hierarchically-structured ZSM-5 zeolite

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and the effect of its physicochemical properties with Cu impregnation on cold-start hydrocarbon trap performance

Heejoong Kim,a,┴ Eunhee Jang,a,┴ Yanghwan Jeong,a Jinseong Kim,a

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Chun Yong Kang,b Chang Hwan Kim,b Hionsuck Baik,c Kwan-Young Lee,a and Jungkyu Choia,*

Department of Chemical & Biological Engineering, Korea University, Seoul 02841, Republic of Korea

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Advanced Catalysts and Emission-Control Research Lab, Research and Development Division, Hyundai

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Motor Group, Hwaseong-si, Gyeonggi-do 18280, Republic of Korea Seoul Center, Korea Basic Science Institute, Seoul 02841, Republic of Korea

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These two authors equally contributed to this work.

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* Corresponding author: Jungkyu Choi

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E-mail address: [email protected], Tel: +82-2-3290-4854, and Fax: +82-2-926-6102

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Graphical abstract

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Highlights

The mesoporosity in SPP particles was increased by removing ethanol and water.

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Mesoporous SPP particles with a Si/Al ratio as low as ~23 could be synthesized.

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Cu-impregnated SPPs were effective for eliminating hydrocarbon (HC) in a cold start.

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The effect of physicochemical properties of SPPs on the HC trap was investigated.

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Cu2+ ions increased HC adsorption, while CuO contributed to HC oxidation.

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Abstract

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A hierarchically structured zeolite (self-pillared pentasil; SPP) comprised of MFI nanosheets or lamellae has been synthesized in various Si/Al ratios and mesoporosities. It turns out that a simple removal of

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ethanol in a synthesis sol resulted in increased mesoporosity, while the additional reduction of water

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further increased mesoporosity. In addition, we could synthesize the SPP particle with the actual Si/Al ratio as low as ~23 with a modest mesoporosity. With these hierarchically structured SPP particles, we

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further conducted copper impregnation on them in order to use as a hydrocarbon (HC) trap. The resulting Cu-impregnated SPPs could not only adsorb HCs in the exit gas stream including water vapor, but also

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serve as an active oxidizer of HCs. Specifically, Cu-impregnated SPP with an actual Si/Al ratio of ~22 and medium mesoporosity exhibited very high performance in cold-start trap tests; desirably adsorbing

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propene and toluene even in the presence of 10 vol% steam, holding them up to higher temperatures (90 °C for propene and 190 °C for toluene), and furthermore, oxidizing the hydrocarbons. The preferred adsorption can be attributed to the larger amount of exchanged Cu2+ ions in SPP particles with a lower Si/Al ratio, while the additional oxidation was due to the CuO particles dispersed on the SPP surface. 3

However, the hydrothermal stability test revealed that the zeolite structure in the Cu-impregnated SPPs was collapsed and transformed into another undesired phase, thus losing the above-mentioned adsorption ability. Nevertheless, the corresponding oxidation performance was well maintained, indicating the robust,

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active role of the CuO particles.

Keywords: Self-pillared pentasil (SPP) particles; mesoporosity; copper impregnation; hydrocarbon trap;

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cold start.

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1. Introduction The regulations on automobile exhaust gas are getting stricter due to their environmentally negative effect [1]. Indeed, many efforts have been made to reduce the amount of hydrocarbon emission

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along with the steady endeavor for the removal of CO, NOx, and so on [2]. The hydrocarbons emitted from the gasoline engines are supposed to be oxidized by three-way catalysts (TWCs) [3, 4]. However, the temperature for the activation of the catalysts (e.g., as reflected by light-off temperature, T50) is usually above 200-300 °C [5-7]. Therefore, it is quite challenging to oxidize and remove the hydrocarbons emitted right after the engine start [1, 3]. This TWC-inactivated period is called as a cold-

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start period, where ~50-80% of the total hydrocarbon emission is known to be directly released into the

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atmosphere [1, 4]. Indeed, several researches targeting to reduce the degree of hydrocarbon emission have

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been conducted [3-22]. One strategy to shorten the time for activating TWCs can be exemplified by the

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location of the catalyst near the engine or heating the catalyst by using electricity [3, 21, 22]. However, the former has a drawback that the catalysts were severely exposed to a very harsh condition and could

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suffer from thermal degradation, while the latter should require additional energy (deeply relevant to the

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engine efficiency) for heating. Another approach includes capturing the hydrocarbons with a hydrocarbon

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adsorbent called hydrocarbon trap (HC trap) until TWCs are thermally activated. In fact, various types of zeolites (e.g., ZSM-5, beta, mordenite, USY, etc.) with or without metal

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impregnation (e.g., Cu, Ag, Fe, Ni, Co, etc.) have been examined for their feasibility as the effective HC trap [5-20]. Among the zeolites tested, many researchers showed that ZSM-5 and beta zeolites are

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promising for the HC trap [5, 7, 15-17]. In evaluating the HC trap performance, multiple techniques such as hydrocarbon adsorption isotherms, breakthrough tests, temperature programmed desorptions (TPDs), and simulated cold-start tests have been adopted in the literature [5-20]. In particular, propene and toluene, representative of hydrocarbon effluents from gasoline engine automobiles, have been used as model 5

compounds in most tests. Through these researches, it was demonstrated that physicochemical properties of zeolites affected the HC trap performances; the adsorption of unsaturated hydrocarbon (i.e., propene) was dependent on the aluminum content of zeolites, while high aluminum content decreased the

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hydrothermal stability of zeolites [5, 9]. Despite the promising results solely with zeolites, they would not provide a full solution package for the cold-start HC trap [7, 9]. Instead, a recent study showed that Cu-exchanged ZSM-5 exhibited an outstanding performance for the cold-start HC trap. The Cu/ZSM-5 HC trap material exhibited almost no emission of propene or toluene at low temperature and even further decomposed the adsorbed

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hydrocarbon compounds at relatively low temperatures (~230-300 °C) [15, 17]. In particular, the

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introduction of mesopores via the sequential combination of dealumination and desilication to the

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conventional microporous ZSM-5 supports helped the resulting Cu-exchanged ZSM-5 to improve the HC

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trap performance, seemingly due to a higher accessibility to the Cu species [17]. A simulation result on Cu-exchanged ZSM-5 showed that Cu2+ ions inside the ZSM-5 framework increased HC trap

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performance by preferring the adsorption of propene [14]. Further researches showed that the

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homogeneously distributed CuO nanoparticles play a predominant catalytic role in oxidizing propene and toluene [17, 18]. Although the Cu-exchanged ZSM-5 particle with mesopores exhibited the superior

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performance in trapping and/or oxidizing propene and toluene effluents, the mesopores were randomly generated and formed through the combined dealumination and desilication. Thus, it was difficult to

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understand the effect of mesopores on the HC trap and conversion performance.

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Therefore, ZSM-5 zeolites equipped with regular mesopores are desirable for elucidating the role,

apparently beneficial role, of the hierarchical structure on the cold-start HC trap. In this study, we adopted the self-pillared pentasil (SPP) supports and impregnated copper species on them; the term of SPP refers to a hierarchical zeolite that consists of self-crossing MFI nanosheets and MEL pillars at the cross-section 6

of the MFI nanosheets. This material possesses regular mesopores in the range of ~2-10 nm due to formation among the neighboring nanosheets (~2 nm in thickness) [23-25]. MFI type zeolites have shown promising performances as acid catalysts for multiple reactions (methanol to olefin, isomerization, and

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catalytic cracking) and as adsorbents for various hydrocarbons [7, 23, 26-31]. The features that high accessibility to active sites through the above-mentioned regular mesopores of the SPP particles can be secured and the SPP particles are easily synthesized made them attractive candidates for good catalysts/supports and adsorbents [23-25, 32, 33].

Despite the promising properties of the SPP particles, we found that an original SPP particle

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with the Si/Al ratio of ~75 was exclusively used in the following works [23, 28, 30]. In this paper, the

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mesoporosities of SPP particles were controlled by changing the ethanol and water content in the

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synthesis sol. Furthermore, a high aluminum content was introduced to the SPP particles with the nominal

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Si/Al ratio as low as ~30. Specifically, the resulting particles were synthesized with a different degree of mesoporosity and chemical composition (i.e., Si/Al ratio) and their morphology, crystallinity, and textural

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properties were intensively investigated. Among the synthesized particles, three SPP samples ((1) with a

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low mesoporosity and high Si/Al ratio, (2) with a high mesoporosity and high Si/Al ratio, and (3) with a medium mesoporosity and low Si/Al ratio) have been subjected to ion exchange to H-form and Cu

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impregnation. This decision was made in order to elucidate the effect of acidity, mesoporosity, and copper catalysts on the HC trap performance. The physicochemical properties of the H-form and Cu-impregnated

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SPPs were assessed and their HC trap performances as well as those of Cu-impregnated SPPs were tested under a simulated cold-start condition. To realize a more realistic simulated condition, the two

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representative hydrocarbons of propene and toluene were fed along with ~10 vol% water vapor. Finally, the long-term stability of Cu-impregnated SPPs was investigated.

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2. Experimental 2.1. Synthesis of SPP particles SPP particles were synthesized according to the procedure described in our earlier study [31]

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adapted from the original method in the literature [23]; a major difference was no use of an aging step before the hydrothermal synthesis in our approach, since it was stated that the aging step did not affect the morphology of the resulting particles [23]. Specifically, a sol for synthesizing SPP particles was prepared by conducting the following steps. First, aluminum isopropoxide (98%, Alfa Aesar) was added to tetraethylorthosilicate (TEOS, 98%, Sigma-Aldrich), and tetrabutylphosphonium hydroxide (TBPOH,

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40%, Alfa Aesar) was added dropwise into the mixture, while stirring. For convenience, this sol is called

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as mixture A. In parallel, sodium hydroxide (NaOH, 98%, Sigma-Aldrich) was added to deionized (DI)

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water and this NaOH solution was added to the already prepared mixture A. The final synthesis sol was

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sealed in a polypropylene bottle and further hydrolyzed at least overnight. The final composition of the synthesis sol was 1 SiO2: x Al2O3: 0.3 TBPOH: 10 H2O: 2x NaOH: 4 EtOH where x = 0.005, 0.01, or

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0.0167 was used for acquiring samples with different Si/Al ratios.

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Compared to the original method in the literature [23], we varied the amount of ethanol and DI

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water in the synthetic sol. For this, the synthetic sol after the hydrolysis step was transferred to a cap-free 45 mL Teflon-liner and a certain portion of ethanol and water was evaporated at room temperature while

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stirring. Along with the above-mentioned molar composition, the molar composition after the removal of ethanol corresponded to 1 SiO2: x Al2O3: 0.3 TBPOH: 10 H2O: 2x NaOH: 0 EtOH, while that after the

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removal of the amount of ethanol and a half of the amount of water corresponded to 1 SiO2: x Al2O3: 0.3 TBPOH: 5 H2O: 2x NaOH: 0 EtOH.

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The hydrothermal reaction was carried out in the Teflon-lined stainless-steel autoclave for 48 h at 115 °C. For completing the reaction, the autoclaves were quenched with tap water. The solid product was recovered by the five repetitions of centrifugation, decanting, and washing with DI water. The

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resulting solid products were further dried at 70 °C at least overnight and calcined at 550 °C for 12 h with the ramp rate of 1 °C/min under the air flow of 200 mL/min. The labels of the samples obtained with the synthesis sols after (1) no evaporation, (2) ethanol evaporation, and (3) additional water evaporation are referred to as α_E1_W1.0, α_E0_W1.0, and α_E0_W0.5, respectively, where α indicates the nominal Si/Al ratio and the numbers next to the letters of E and W stand for the fractions of ethanol and water with

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respect to the original sol.

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Among the calcined nine different Na-form SPP particles, 100_E1_W1.0, 100_E0_W0.5, and

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30_E0_W0.5 samples were further turned into H-form counterparts in order to elucidate the effect of their

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acidity and mesoporosity on the cold-start hydrocarbon trap performance. For this, each sample was stirred in 1 M ammonium nitrate (NH4NO3) solution at 80 °C for 6 h at a fixed ratio of 0.01 Na-form SPP

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particles (g)/NH4NO3 solution (mL), and the resulting ionic-exchanged samples were recovered by three

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repetitions of centrifugation, decanting, and washing with DI water. The recovered samples were dried at 70 °C at least overnight and calcined at 500 °C for 6 h with the ramp rate of 10 °C/min under air flow of

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200 mL/min [34]. The resulting particles were denoted as L_100, H_100, and M_30, respectively, where the letter H, M, and L stand for the high, medium, and low mesoporosities in the resulting particles,

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respectively, and the numbers in the end represent the nominal Si/Al ratios. Furthermore, Cu was impregnated on the H-form SPP particles by using a wetness impregnation method. Specifically, copper

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nitrate trihydrate (Cu(NO3)2∙3H2O, 98%, Sigma-Aldrich) was dissolved in DI water to make 0.04 M Cu(NO3)2 solution. Then, a certain amount of H-form SPP particles was added to the copper nitrate solution, targeting for 5 wt% Cu impregnation. After that, the mixture was placed in a rotary evaporator to remove all of water. After completing the water removal, the Cu-impregnated SPPs were recovered, dried 9

at 100 °C for 3 h, and finally calcined at 550 °C for 6 h with the ramp rate of 1 °C/min under air flow of 200 mL/min. The resulting particles are denoted as Cu/L_100, Cu/H_100, and Cu/M_30, respectively, where Cu indicates the copper impregnation on the H-form SPP particles (L_100, H_100, and M_30). In

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addition, commercially available NH4-form ZSM-5s with the Si/Al ratios of 75 (CBV 1502, PQ Corporation; manufactured by Zeolyst International) and 140 (CBV 28014, Zeolyst International) were used as references. All as-received ZSM-5 samples were calcined at 500 °C for 6 h with the ramp rate of 10 °C/min under air flow of 200 mL/min to obtain the H-form ZSM-5 sample. Cu was further impregnated onto the H-form ZSM-5 particles using the same method described above. For convenience,

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H-form and Cu-impregnated ZSM-5 samples were denoted as C_x and Cu/C_x, respectively, where C

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represents commercial zeolites and x indicates their Si/Al ratios (75 or 140).

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2.2. Characterizations

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Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained by using a Hitachi S-4800 field emission scanning electron microscope (FE-SEM) and a Tecnai

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G2 F30ST field emission transmission electron microscope (FE-TEM), respectively. The energy

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dispersive X-ray spectroscopy (EDX) data were obtained by using a Hitachi SU-70 field emission

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scanning electron microscope. High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images and EDX data were obtained by using a FEI XFEG-Titan themis3 Double Cs &

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Mono. transmission electron microscope. Rigaku Model D/MAX-2500V/PC with RINT2000 vertical goniometer was used to obtain X-ray diffraction (XRD) patterns of calcined zeolite samples based on a

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Cu Kα radiation (40 kV, 100 mA, λ=1.54 Å). The simulated XRD patterns of the MFI zeolite, CuO, and αcristobalite phases were generated from the corresponding crystallographic information files (CIF) by using the Mercury software (ver. 3.8, available from the Cambridge Crystallographic Data Centre website). All three CIF files were provided by the Material Studio 7.0 (Accelrys). N2 physisorption 10

isotherm data at the temperature of 77 K were obtained by using a Micromeritics ASAP2020 system. Pore sizes and volumes were calculated based on a BJH method provided by the manufacturer. In order to estimate surface areas of each sample at different length scales, we referred to the BET analysis and a

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modified t-plot method (recently reported in the literature [35]). Fourier transform infrared spectroscopy (FT-IR) was used to locate and quantify the acid sites in SPP particles by using two base molecules of pyridine and 2,6-di-tert-butylpyridine. Self-pelletized samples were first activated at 500 °C under vacuum for 6 h in an in-situ FT-IR cell. After cooling down to 150 °C, the FT-IR spectra of the activated samples were acquired after removing weakly attached Py or dTBPy molecules under 60 min vacuuming;

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pyridine (Py; saturated vapor pressure of 2.80 kPa at 25 °C) or 2,6-di-tert-butylpyridine (dTBPy;

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saturated vapor pressure of 0.034 kPa at 25 °C) was adsorbed on the samples by flowing the saturated

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vapor carried by 30 mL/min He flow for 1 h [36]. Since the kinetic diameter of Py and dTBPy are estimated to be 0.55 nm and 1.05 nm, respectively, the total acid sites and the Brønsted acid sites located

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outside the microporous area (i.e., external and mesoporous surface areas) of MFI zeolites (pore size =

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~0.55 nm) can be analyzed by using the FT-IR spectra of each molecule [37, 38]. To quantify the amount

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of acid sites, the FT-IR spectra of the wavenumbers of 1450 cm-1 (Py), 1550 cm-1 (Py), and 1615 cm-1 (dTBPy) were used to calculate the concentration of total Lewis acid sites, total Brønsted acid sites, and

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Brønsted acid sites located on the mesoporous and external surface areas, respectively. For convenience, the Lewis acid and Brønsted acid sites are denoted as L and B sites, respectively. After completing the

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cold-start test (described in section 2.3), the tested samples were heated up to 900 °C with a 5 °C/min ramp rate under air flow of 100 mL/min to confirm the formation of coke via the thermogravimetric

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analysis (TGA, Q50, TA Instruments).

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2.3. Simulated cold-start tests (CST) All experiments were performed in a fixed bed configuration; ~0.06 g of each sample, which had been sieved to lie in the range of 150-250 μm, was packed into a quartz tubular reactor (inner diameter:

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6.9 mm and outer diameter: 9.6 mm). The temperature of the fixed bed was controlled by using a temperature controller (UP35A, Yokogawa), and the flow rate of feed streams was controlled by using mass flow controllers (High Tech, Bronkhorst). The temperature was measured by using a thermocouple below the quartz frit on top of which the sample was placed. The steam was generated by injecting a fixed rate of liquid water into heated tubings. After passing the reactor, a trap, around which cold water (~1 °C)

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was circulated, was placed in an effort to remove the steam in the outlet gas stream. The outlet line after

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the trap was connected to a mass spectrometer (Lab Questor-RGA, Bongil). All the samples (i.e., the

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calcined H-form and Cu-impregnated SPP particles) were activated under He flow of 30 mL/min at

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600 °C for 30 min. In CSTs, ~100 mL/min of a gas mixture that comprised 100 ppmv propene, 100 ppmv toluene, 1 vol% O2, 10 vol% H2O, and 560 ppmv Ar (500 ppmv Ar for commercial ZSM-5s with and

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without impregnated Cu) balanced with He was fed to the activated sample, resulting in WHSV =

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100,000 mL∙g-1∙h-1. In the feed, Ar was included to serve as an internal standard for quantification of the molar composition in the gas outlet based on the mass spectrometry. The measurement of CSTs started at

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70 °C and the reactor was maintained at that temperature for 5 min. Then the reactor was heated up to 600 °C with the ramp rate of ~50 °C/min and soaked at 600 °C for 30 min. The signals corresponding to

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m/z = 40 for Ar, 42 for propene, 91 for toluene, 32 for O2, and 18 for H2O were detected to see any difference with respect to the feed components, while the signals corresponding to m/z = 28 for CO and

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44 for CO2 were also monitored to detect any oxidation process. In addition, the signals corresponding to m/z = 56, 77, and 106 were recorded in order to track production of other hydrocarbons, which were seemingly side products generated from propene and/or toluene [6, 39]. To test the stability of Cuimpregnated SPPs, the three consecutive CST performances of a fresh sample and the CST performances 12

of hydrothermally treated Cu-impregnated SPPs were conducted. For reliable measurements in the three consecutive CST performances, the absence of steam in the flow reactor was ensured between the CST tests (i.e., while cooling down the reactor from 600 °C back to 70 °C) by flowing ~100 mL/min of He for

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~6 h. The Cu/L_100, Cu/H_100, and Cu/M_30 were hydrothermally treated under 100 mL/min of He with ~10 vol% steam at 800 °C for 24 h, and the resulting samples were referred to as Cu/L_100 HT, Cu/H_100 HT, and Cu/M_30 HT, respectively, where HT represents the hydrothermal treatment. The CST performances of the hydrothermally treated Cu-impregnated SPPs were investigated under the identical

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condition mentioned above.

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3. Results and discussion

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3.1. Effect of ethanol and water content on the synthesis of SPP particles

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3.1.1. Crystal structures

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At first, we conducted a XRD analysis on the particles in order to identify their crystallinity (Fig.

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1). The XRD patterns of all the samples showed that they have a crystal structure, except for 30_E1_W1.0 which appeared to be an amorphous material. Along with previous studies on the SPP zeolites [23-25], the

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elaborate comparison of the XRD patterns of three SPPs (synthesized with the nominal Si/Al ratio of 100; Fig. S1) with the simulated XRD patterns of MFI and MEL zeolites also supported that all SPP particles

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except for 30_E1_W1.0 had the MFI structure, not MEL structure. However, despite the verified crystallinity, the SPP particles had some XRD peaks missing. Specifically, the XRD peaks corresponding

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to the general (hkl) planes were weakened, while those corresponding to the (h0l) or (0k0) planes were pronounced. This could be ascribed to the large ac-plane of the nanosheets in SPPs and/or preferred alignment of the large ac-plane with respect to a sample holder during measurements. As discussed in our previous study [31], the characteristic XRD pattern comprised of the specific peaks corresponding to the 13

(h0l) or (0k0) planes indicates formation of the thin layers of the MFI nanosheets along the b-axis [31]. Previous studies on the synthesis of mesoporous MFI zeolites reported that the decrease in ethanol content in a synthetic precursor led to formation of thinner layers in MFI zeolites, given the use of TEOS

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as a silica source [40, 41]. Also, it has been reported that ethanol could work as a template for the synthesis of MFI zeolites [42, 43]. Based on these results, we speculate that ethanol contributed to the crystal growth along the b-axis in SPP particles, and thus the removal of ethanol accommodated the reduction of the nanosheet thickness and the preferred formation of SPP particles with the thinner nanosheets.

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3.1.2. Morphologies

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Fig. 2 shows that the morphologies of the synthesized particles were significantly affected by the

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change in the molar composition of the synthetic sol. In particular, the amount of ethanol (generated due

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to the hydrolysis of TEOS) and water in the synthetic sol influenced the synthesis of SPP particles.

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100_E1_W1.0 particles had size of ~400-600 nm and exhibited non-smooth and rather bumpy surface (Fig. 2a1). Each particle of 100_E0_W1.0, obtained after the removal of ethanol, was primarily

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comprised of thinner or sharper nanosheets, and the particle size decreased (Fig. 2a2). The morphology of

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the 100_E0_W0.5 samples, which were synthesized after additional removal of water, was similar to those of 100_E0_W1.0 in terms of the sharp nanosheets, and the size further decreased (Fig. 2a3). In this

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study, EDX analyses, whose results were already shown to be comparable to the results obtained from 29

Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectra [31], were adopted for

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estimating the Si/Al ratios of the zeolite particles. Furthermore, since the XRD patterns of the resulting SPP particles (Fig. 1) showed their relatively low crystallinities, it was desirable to acquire the actual Si/Al ratios of SPP particles by measuring multiple points in crystal-like particles in the EDX measurements, as compared to the bulk-scale measurements including inductively coupled plasma (ICP) 14

[44-47]. Among the three samples synthesized at the Si/Al ratio of 100, 100_E0_W1.0 and 100_E0_W0.5 were almost identical to the original SPP particles in the literature [23-25], though the Si/Al ratio of 100_E0_W1.0 showed a larger value (~217) than the reported value of ~75 (Tables S1-S2). The large

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standard deviation in the Si/Al ratio of 100_E0_W1.0 indicates a difficulty in uniformly incorporating Al atoms into the MFI framework (Table S2). However, a particle synthesized after further evaporation of water (i.e., 100_E0_W0.5) exhibited a Si/Al ratio of ~51 with a relatively small standard deviation. The different Si/Al ratios among the three samples (Table S2) suggest a complexity of the formation of a hierarchical structure. Along with the variation of the ethanol and water content, we also changed the

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Si/Al ratio in the synthetic sol. In fact, the SPP particles were reported with the actual Si/Al ratio as low

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as ~75 in the literature [23, 28, 30].

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For the synthesis with the nominal Si/Al ratio of 50, the surface of the resulting all three particles

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(50_Ex_Wy; (x, y) = (1, 1.0), (0, 1.0), and (0, 0.5)) was rather close to that of 100_E1_W1.0 under the SEM resolution (Fig. 2b1-b3). The corresponding Si/Al ratios lied in the range of ~31-86, which deviated

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from the nominal value of 50 (Table S2), similar to the sample series obtained with the nominal Si/Al

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ratio of 100. In particular, the surface morphology due to the aggregation of sharp nanosheets, analogous to the jungle gym in a playground, was not clearly observed at the SEM images. This indicates that the

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synthesis toward the SPP particle was a sensitive function of the Si/Al ratio and apparently requires a high Si/Al ratio. We further decreased the nominal Si/Al ratio down to 30. Among the three particles

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(30_Ex_Wy; (x, y) = (1, 1.0), (0, 1.0), and (0, 0.5)), only 30_E0_W1.0 was close to the particles synthesized with the Si/Al ratio of 50 (Fig. 2b1-b3). On the contrary, both 30_E1_W1.0 and 30_E0_W0.5

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seemed to comprise aggregated small irregular particles, with a smooth surface for 30_E1_W1.0. Furthermore, the Si/Al ratio of 30_E1_W1.0 was 4.1, while those of 30_E0_W1.0 and 30_E0_W0.5 were 25 and 23, respectively, close to the nominal value of 30 (Table S2). The different morphology and Si/Al ratio of 30_E1_W1.0 suggest that it did not possess the MFI crystal structure. 15

In general, the ethanol content, generated by the hydrolysis of the silica source (here, TEOS), can be negligible due to its very little fraction in a synthetic precursor [48-50]. Therefore, how to deal with ethanol and thus, its amount has not been provided or described carefully. This tendency was found

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for the synthesis of SPP particles. Through the intensive investigation of several papers that report the synthesis of SPP particles, we recognized that the amount of ethanol was not clearly described [23, 24]; see details in Table S1 in the Supporting Information. However, considering the molar composition of 1 SiO2: 4 EtOH: 10 H2O in the original synthesis sol for SPP particles, we should not neglect the effect of ethanol on the synthesis of MFI zeolite particles [42, 43, 51]. From the observation of a change in the

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surface morphology at different molar compositions (especially, the amount of ethanol and water), it can

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be concluded that at the high Si/Al ratio, the existence of ethanol led to the formation of conventional

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ZSM-5 type particle only with a minor difference of the rough surface, while the removal of ethanol and water induced the SPP structure. However, the additional decrease of the nominal Si/Al ratio from 100

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through 50 to 30 weakened the SPP feature, though the rough surface was well preserved. In addition,

D

considering the lowest Si/Al ratio reported in the literature was ~75 [23, 28, 30], our attempt to reduce the

TE

Si/Al ratio down to ~30 will expand the utility of the SPP particles in a catalytic field. The morphological change according to the change in molar composition was further

EP

investigated with the TEM analysis (Fig. 3). The morphologies of 100_Ex_Wy ((x, y) = (1, 1.0), (0, 1.0), and (0, 0.5)) samples, observed in Fig. 2, were much clearer in the respective TEM images (Fig. 3a1-a3).

CC

Although 100_E1_W1.0 was primarily comprised of thick nanosheets (Fig. 3a1), 100_E0_W1.0 obtained after the evaporation of ethanol clearly had the sharp MFI nanosheets or lamellae whose thickness

A

decreased significantly (Fig. 3a2). The thickness of both particle size and nanosheet size was further decreased with the additional removal of water (Fig. 3a2-a3), as observed in the SEM results (Fig. 2a2a3). In the case of samples synthesized with the nominal Si/Al ratios of 50 and 30, we recognized that all the particles were comprised of the nanosheets, except for 30_E1_W1.0 (Fig. 3c1). The series of 16

50_Ex_Wy were almost comparable to the series of 100_Ex_Wy; the nanosheet feature was monotonically increased with the removal of ethanol (Fig. 3b2) and ethanol/water (Fig. 3b3). Additional decrease of the nominal Si/Al ratio down to 30 led to the formation of the thicker nanosheets as shown in

SC RI PT

Fig. 3c2-c3. The smooth, smaller globular particles in 30_E1_W1.0 (Fig. 3c1) suggest the formation of another phase, which was different from the MFI zeolite phase. The combined SEM and TEM characterizations revealed the reproducible self-pillaring especially in the absence of ethanol, which had been inevitably produced due to the hydrolysis of TEOS. 3.1.3. Textural properties

U

N2 physisorption isotherms at 77 K have been measured to analyze the pore structures of the SPP

N

particles (Fig. S2 and Table S3). From the relative pressure range of ~0.4-0.8 in the N2 physisorption

A

isotherms, it appears that regardless of the Si/Al ratio, the removal of ethanol and the additional removal

M

of water in the synthetic sol helped to increase mesoporosity (Fig. S2a-c). This was supported by the

D

increased mesoporous region in the BJH pore size distribution (Fig. 4a-c). In the SPP particles, the mesopores are formed among the self-pillared MFI nanosheets or lamellae [23]. Thus, the degree of the

TE

mesopores in the resulting SPP particles, especially in the range of ~2-10 nm, was likely induced by the

EP

decrease in the thickness of the plate-like nanosheet and corresponding increase of the spaces between nanosheets observed in the SEM and TEM images (Figs. 2-3). Also, the samples with a higher Si/Al ratio

CC

exhibited more intense mesoporosity after removing ethanol and ethanol/water content in the synthesis sol (Fig. 4b-c). This can also be attributed to the thicker or chopped nanosheets in SPP particles with the

A

lower Si/Al ratios (50 or 30) as observed in TEM images (Fig. 3). The surface areas and pore volumes calculated from the N2 physisorption isotherm also supported the increase in mesopores after removing ethanol and ethanol/water content (Table S3).

17

3.2. H-form and Cu-impregnated SPP particles 3.2.1. Physicochemical properties Among the eight different SPP particles described in section 3.1, we chose three different types

SC RI PT

of SPP particles, while considering the Si/Al ratio and mesoporosity. The three SPP particles were converted to H-form counterparts and then, copper was impregnated on them. The normalized XRD patterns of L_100, H_100, and M_30 in Fig. 5 showed that all three SPP particles, now ionic-exchanged with protons, maintained their original MFI type zeolite structure shown in Fig. 1. In addition, Cuimpregnated samples also showed the intact MFI zeolite structure, suggesting that the SPP structure was

U

well preserved during the processes relevant to the ionic-exchange to H-form and Cu impregnation.

N

Furthermore, Cu-impregnated samples exhibited the XRD peaks corresponding to the CuO phase (around

A

36° and 39°), and the corresponding size was estimated to be 20 nm based on the Scherrer equation (Fig.

M

S3). Assuming that the CuO particles were randomly oriented, the amount of ~20 nm sized CuO particles among the three samples can be simply estimated from the corresponding XRD peak area. Specifically,

D

the relative areas under the XRD peak of the (002) plane of CuO phases were ~0.6, ~1, and ~0.4 for

TE

Cu/L_100, Cu/H_100, and Cu/M_30, respectively. Considering that the impregnated Cu species will exist in a form of CuO, the amounts of the ~5 nm sized CuO particles were large in decreasing order of

EP

Cu/M_30, Cu/L_100, and Cu/H_100.

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The SEM and TEM images in Figs. 6-7 confirmed that the morphologies of the SPP supports after carrying out the ion exchange to H-form and the consecutive Cu impregnation were almost identical

A

to those of the Na-form counterparts (Figs. 2-3). The Si/Al ratios of both H-form and Cu-impregnated SPP particles were also comparable to those of Na-form particles except L_100 and Cu/L_100 (Tables 1 and S2). For L_100 and Cu/L_100, it was quite challenging to obtain a reliable Si/Al ratio value as reflected by the high standard deviation value as relative to the averaged value. This indicates a minor 18

portion of Al atoms incorporated into the zeolite framework and thus, a higher Si/Al ratio. The Na/Al ratios of all three H-form samples demonstrate that Na+ ions were thoroughly exchanged by H+ ions, though we cannot rule out a possible presence of Na+. Regarding the Cu wt%, all Cu-impregnated SPPs

SC RI PT

had smaller Cu wt% (~3-4 wt%) than the nominal value of 5 wt%, which was consistent with previous studies [52, 53]. For Cu/L_100 and Cu/H_100, the Cu wt% values measured at multiple particles were comparable to those obtained from the SEM/EDX mapping (Fig. S4), validating that all Cu species were placed in and on the SPP supports. On the contrary, Cu/M_30 had ~5.6 Cu wt% from multiple particles, which was higher than the value obtained from the SEM/EDX mapping (~3.5 Cu wt%). Under the limited

U

SEM resolution, it appears that Cu species were not homogeneously distributed on the particles, as

N

concentrated Cu species (reflected by intense red color) were observed in the SEM/EDX mapping (Fig.

A

S4).

M

In particular, the nanosheet constituents of the SPP particles were well-preserved after exchange to H-form as supported by the high resolution TEM analysis (Fig. 7a1-a3); nanosheets (in L_100 and

D

H_100) and fragmented or chopped nanosheets (in M_30). For the Cu-impregnated SPPs, particles with a

TE

size of ~20 nm were sporadically observed (as indicated by black arrows in Fig. 7c1 and Fig. S5), while ~5 nm sized small particles were more often observed and well distributed on the SPP particles in the

EP

TEM images and STEM images (Figs. 7c1-c3 and 8). The maximum Cu wt% achievable by the ion exchange in MFI zeolites with the actual Si/Al ratios of ~61, ~82, and ~24 will be 0.8, 0.6, and 2.1 wt%,

CC

respectively. Furthermore, the excessive amount of Cu atoms (to be ~3-4 wt % in the final portion) was provided for the Cu impregnation. Therefore, it is reasonable to accept that the small particles observed

A

on the surface possessed the CuO phases. However, the chemical mapping reveals that the distribution of Cu atoms was found to be uniform and continuous throughout the SPP particles rather than isolated in a form of particles (Fig. 8). Considering the limited mapping resolution in Fig. 8, it appears that the Cu atoms were present in the form of the above-mentioned particles on the SPP surface and cations in and on 19

the SPPs. A thorough TEM analysis suggested that the 20 nm sized particles would not be representative of a majority of CuO particles; the particles in the size of ~5 nm were found to be predominant on the surface (Fig. 7) along with the minor observation of the 20 nm sized or larger ones (Figs. 7c1-c3 and S5).

SC RI PT

In fact, the XRD technique is not able to identify the presence of ~5 nm sized CuO particles supported on the SPP particles [54, 55]. In addition, the SEM/EDX results (Fig. S4), which provide the detection scale between those of XRD (bulk scale) and TEM (super-microscopic scale) analyses, demonstrate that the 20 nm sized CuO particles would be primarily placed on the SPP surface, also validated by Cu wt% obtained from the SEM/EDX results (Table 1). Therefore, it is reasonable to conclude that both the 5 nm and 20

U

nm sized CuO particles were present on the SPP surface. Nevertheless, the minor existence of isolated

N

CuO particles away from the SPP particles cannot be completely ruled out.

A

The N2 physisorption isotherms and pore size distributions also supported the preservation of the

M

original textural properties in H-form SPP particles after the Cu impregnation. The microporous surface area did not significantly change after Cu impregnation (~90-105% compared to the H-form SPPs),

D

though the mesoporous and external surface area decreased to some extent (~8% for L_100 and ~17-29%

TE

for H_100 and M_30) (Figs. 9 and S6 and Table 2). This reduction in the mesoporous region was pronounced for the SPP with higher mesoporosity and presumably due to the formation of the above-

EP

mentioned 5 nm sized CuO particles on the mesoporous surface of SPP particles.

CC

In the process of Cu impregnation onto the H-form ZSM-5, Cu2+ ions can partially replace the protons in Si(OH)Al (directly relevant to the B sites), and concomitantly, the corresponding B sites

A

decrease [56-58]. As can be seen in Table 2, the B sites in all three H-form SPP particles (L_100, H_100, and M_30) were mainly located at the mesoporous and external surface area. Specifically, among the three samples, the amount of total B sites was the largest in M_30 (191 μmol/g; mainly due to the lowest Si/Al ratio) and the smallest for L_100 (45 μmol/g; mainly due to the lowest mesoporous and external 20

surface areas). After carrying out the Cu impregnation process, the decrease in the total B sites was highly pronounced for Cu/M_30 (69 μmol/g) and non-significant for Cu/L_100 (24 μmol/g), supporting that the amounts of the Cu2+ ions in the Cu-impregnated SPP particles were largest for Cu/M_30 and smallest for

SC RI PT

Cu/L_100. This result indicates that the incorporation of Cu2+ ions with the MFI zeolite structure was effective in SPP particles with a lower Si/Al ratio, while protons were ionic-exchanged to Cu2+ ions [59]. Along with the decrease in the total B sites, the decrease in the external B sites was also pronounced for Cu/L_100 and Cu/H_100, indicating that most of Cu2+ ions were located at the mesoporous and external surface. For Cu/M_30, it appears that the decrease in external B site suggested that Cu2+ ions were more

U

located at the internal surface than at the mesoporous and external surface.

N

In order to elucidate the effect of the textural properties, Cu-impregnated commercial zeolite

A

samples (Cu/C_75 and Cu/C_140) were used. The particle sizes of C_75 and C_140 were 0.8-1 μm and 1-

M

2 μm, respectively (Fig. S7a1-a2). The morphologies, crystal structures, and textural properties of the ZSM-5 supports were well maintained after Cu impregnation (Figs. S7-S9). The measured Si/Al ratios of

D

the samples were 93.0 and 94.2 for C_75 and Cu/C_75 and 210 and 193 for C_140 and Cu/C_140,

TE

respectively. The copper weight percents were measured to be ~5.5-5.8 wt% (Table S4). The large standard deviation of Cu wt% in Cu/C_75 can be attributed to the poor CuO dispersion, as indicated by

EP

red arrows in the SEM/EDX mapping (Fig. S10). For C_75 and C_140, they had primarily micropores (Table S5), while the SPP particles possessed a noticeable amount of mesopores (Table 2). For the acid

CC

sites, C_75 had 150 μmol/g of B sites, comparable to that of M_30, while C_140 had 68 μmol/g, comparable to that of L_100. Most B sites in C_75 and C_140 were mainly located at the micropores

A

(Table S5). After carrying out the Cu impregnation process, the amount of B sites decreased to 61 μmol/g for Cu/C_75 (in turn, 89 μmol/g for Cu2+ ions) and 41 μmol/g for Cu/C_140 (in turn, 27 μmol/g for Cu2+ ions). From the quantification of B sites via Py and dTBPy probe molecules, it was found that Cu2+ ions in Cu/C_75 and Cu/C_140 were primarily located at the micropores as expected. 21

We found that along with the decrease in the total B sites, L sites drastically increased about ~45 times in all Cu-impregnated SPPs (Table 2), while the L sites were increased by ~2-3 times in Cuimpregnated commercial ZSM-5 zeolites (Table S5). It was already reported that L sites drastically

SC RI PT

increased and at the same time, B sites decreased, as Cu2+ ions were incorporated with the ZSM-5 zeolite framework [56-58]. In previous studies, it was shown that one mole of Cu2+ ion in ZSM-5 was able to adsorb two moles of pyridine due to spatial constraint, while that in Y zeolite could adsorb up to four moles of pyridine [60, 61]. On the contrary, one Cu2+ ion is known to adsorb up to 5 molecules of pyridine in the absence of spatial constraint [62]. In the case of SPP particles, the B sites, which were now

U

replaced by Cu2+ ions, mainly existed in the mesopore and external surface area for Cu/L_100 and

N

Cu/H_100 and in the microporous surface for Cu/M_30 (Table 2). On the contrary, the amount of pyridine

A

adsorption on Cu2+ ions (here, reflected by the increase of L sites/decrease of B sites) suggested that Cu2+ ions were primarily located at the mesoporous and external surface by replacing the protons in the

M

external B sites for all three samples. Then, we realized that Cu/M_30 would have an inconsistent

D

conclusion regarding the location of Cu2+ sites based on (1) acid site titration via Py and dTBPy

TE

molecules and (2) the above-mentioned ratio, the increase of L sites/decrease of B sites. At this point, the quantification and location of Cu2+ ions based on the increase of L sites/decrease of B sites ratio appears

EP

to be less reliable, as the corresponding values for the three Cu-impregnated SPPs (~4-5) were not considerably different from those for the Cu-impregnated microporous C_140 and C_75 (~2-3). Therefore,

CC

it would be reasonable to accept that for Cu/M_30, Cu2+ ions, formed by replacing B sites, were likely to be located both at the micropores and at the mesoporous and external surface. Nevertheless, Cu 2+ ions in

A

Cu/M_30 were considerably located at the mesoporous and external surface, as compared to Cu/C_75 and Cu/C_140; the degree of decrease in B sites, titrated by dTBPy (22 μmol/g), was ~30% with respect to that titrated by Py (69 μmol/g). Although some researchers claimed CuO as a Lewis acid [63], it is

22

reasonable to consider that CuO mainly acts as base [64-66]. In this sense, a possible contribution of the CuO particles to increasing the L sites (Table 2) can be disregarded.

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3.2.2. CST performance of H-form SPP particles Figs. 10-11 show the CST performances of H-form and Cu-impregnated SPPs; (1) propene, toluene, and oxidized species (CO2 and CO) (Fig. 10) and (2) plausible side products (Fig. 11) in the outlet stream were monitored as a function of time. For the H-form SPP, L_100, H_100, and M_30 showed almost identical outlet profiles. Specifically, Fig. 10a1 reveals that none of H-form SPPs adsorbed propene almost from the beginning, apparently due to the inhibition of preferred/strong adsorption of

U

steam (10 vol% in the feed). One minor difference was observed for H_100; some of propene were likely

N

consumed right after temperature rise and recovered back to the feed concentration (Fig. 10a1). On the

A

contrary, Fig. 10a2, the three H-form SPPs could hold up toluene until the temperature was increased up

M

to ~140 °C, after which rapid desorption occurred within ~2-3 min. It appears that all three samples

D

desorbed all of the adsorbed toluene. It was noted that above 300 °C, both propene and toluene passed through all three samples without any adsorption (Fig. 10a1-a2). Adsorption of propene and toluene in

TE

zeolites is known to increase with the increased surface area and especially, with the increased acid site [5,

EP

12, 13, 19, 67]. When propene and steam adsorb on H-form ZSM-5 zeolites at the same time [13, 68], they will compete for the adsorption sites [14]. Under this circumstance, the excessive amount of steam

CC

compared to propene (10% steam vs. 0.01% propene in this study) made propene difficult to settle down on the adsorption sites (Fig. 10a1). However, in all three H-form SPPs, differences in their surface areas

A

and acid site concentrations (summarized in Table 2) did not make any discernible difference in the coldstart trap performances with respect to both propene and toluene. None of CO2 or CO evolved, indicating no oxidation of hydrocarbons (here, propene and toluene) (Fig. 10a3).

23

Fig. 11a1-a3 shows that other hydrocarbons plausibly resulted from non-oxidative conversion of propene and/or toluene in H-form SPP particles. Specifically, the m/z = 56 (Fig. 11a1) was reported to correspond to 2-methylbutane relevant to propene oligomerization [39], while the m/z = 77 (Fig. 11a2)

SC RI PT

and 107 (Fig. 11a3) were likely to be corresponding to benzene and xylene isomers (possibly including benzaldehyde) relevant to toluene disproportionation [6]. In H-form SPPs, only H_100 generated some molecules corresponding to m/z = 56, while all H-form SPPs showed hydrocarbons corresponding to the m/z = 77 and 106. Despite our substantial investigation of this unexpected appearance for H_100, its origin was not clear, as the current test protocol that required mixtures of propene, toluene, steam, and

U

oxygen rather resulted in the convoluted phenomenon. Nevertheless, we speculate that oligomers relevant

N

to the MS signal of m/z = 56 was ascribed to the catalytic activity of the vacant B sites, which were now

A

formed by toluene desorption. We recognized that when such MS signal of m/z = 56 appeared suddenly, propene emission profile decreased and soon increased back in 5-9 min (Fig. 10a1) and at the same time,

M

toluene was desorbed (Fig. 10a2). Under this circumstance, the B sites that became vacant after toluene

D

desorption can possibly adsorb propene molecules quickly and catalyze them toward the oligomers

TE

(relevant to MS signal of m/z = 56). In addition, it was noted that the emission peaks due to the toluene disproportionation were observed almost at the identical time when toluene was desorbed and released

EP

(Fig. 10a2). To sum up these features, all three H-form SPPs (L_100, H_100, and M_30) did not have any ability to retard the emission of propene and toluene up to the activated temperature of TWCs and to

CC

oxidize the hydrocarbons. These features indicate that pristine H-form SPPs themselves are not suitable for the cold-start trap.

A

3.2.3. CST performance of Cu-impregnated SPP particles Cu-impregnated SPPs showed different, unique emission profiles (Fig. 10b1-b3) compared to the pristine H-form SPPs (Fig. 10a1-a3). Notably, all three Cu-impregnated SPPs could adsorb propene at the 24

initial temperature of 70 °C, with a highest adsorption ability being observed with Cu/M_30. Specifically, Cu/L_100 first started to emit propene even before the heating started, while Cu/M_30 could desorb propene around 90 °C. Cu/H_100 showed performance in-between. For toluene emission profiles (Fig.

SC RI PT

10b2), all three Cu-impregnated SPPs could adsorb toluene completely, as similar to H-form counterparts. One big difference was that the desorption of toluene was most retarded for Cu/M_30 (toluene started to be emitted at 190 °C). Such good performance of Cu/M_30 for toluene can be supported by the stronger adsorption in favor of propene shown in Fig. 10b1. Even, Cu/L_100 showed the improved adsorption ability compared to the H-form counterpart (L_100). However, interestingly, the toluene desorption

U

behavior in Cu/H_100 was almost identical to that in H_100, though the desorbed amount was much

N

decreased for Cu/H_100. It appears that Cu/L_100 desorbed most of the adsorbed toluene (as indicated by

A

the bigger area), while the amount of desorbed toluene was significantly smaller for both Cu/H_100 and Cu/M_30. In particular, the features of the latest toluene desorption and its much reduced amount

M

rendered Cu/M_30 highly desirable for the effective HC trap.

D

The decreased amount of desorbed propene and toluene was correlated with their active

TE

conversion into CO2 and CO (Fig. 10b3) and/or other hydrocarbons (Fig. 11b1-b3). All three Cuimpregnated SPPs could eliminate all the feed components about 600 °C apparently due to the effective

EP

oxidation to CO2 or CO. Specifically, evolutions of CO2 and CO appeared at ~200 °C and drastically increased in increasing order of Cu/L_100, Cu/H_100, and Cu/M_30 (at 300 °C, 350 °C, and 370 °C,

CC

respectively). In particular, it appears that the later desorption of propene and toluene led to the later oxidation process, indicating the beneficial role of Cu/M_30 as an effective HC trap. Carbon balances

A

were estimated to be 100 ± 3% from the summation of the amount of (1) adsorbed propene and toluene and (2) produced CO2 and CO (quantified with a pre-calibration curve), indicating that all hydrocarbons above 300 °C were fully converted to CO2 or CO. The carbon balance measurement with TGA analysis up to 800 °C supports that the coke was rarely formed on the Cu-impregnated SPPs (Fig. S13). 25

Considering that H-form SPPs showed no differences in propene, toluene, and CO2 and CO profiles (Fig. 10a1-a3), the different emission profiles in Cu-impregnated SPPs could be ascribed to new physicochemical properties related to the Cu impregnation (in the form of Cu2+ ions and CuO particles).

SC RI PT

In previous studies, it was reported that propene was chemically and strongly adsorbed onto Cu-ZSM-5, showing significantly increased chemisorption compared to that onto H-ZSM-5 [15, 69]. Indeed, a molecular simulation study on the adsorption of propene and toluene in Cu-ZSM-5 while considering competitive adsorption of steam revealed that propene was mainly located at Cu2+ ions present inside ZSM-5 [14]. Especially, among the Cu2+ ions incorporated with the ZSM-5 framework, Cu2+ ions

U

replacing protons in B sites played a critical, beneficial role on increasing the adsorption affinity toward

N

propene [59]. Based on these studies, the decrease in the total B sites (plausibly, external B sites) after carrying out the Cu impregnation process (Table 2) will give a clue to understand the propene emission

A

profile shown in Fig 10b1. Among the three Cu-impregnated SPPs, the largest amount of the Cu2+ ions in

M

Cu/M_30 increased propene affinity and enabled propene to be adsorbed even in the presence of 10 vol%

D

steam at 70 °C and desorbed at a higher temperature (90 °C). In this sense, the earliest propene desorption

TE

in Cu/L_100 could also be ascribed to the smallest amount of the Cu2+ ions. In addition, toluene is reported to prefer to be adsorbed toward Cu2+ ions [20]. Although propene

EP

adsorption was linearly increased with the increased amount of Cu2+ ions (Fig. 10b1), the amount of Cu2+ ions could not account for the emission profile of toluene in Cu-impregnated SPPs (Fig. 10b2). This non-

CC

linear correlation between the toluene desorption and the amount of Cu2+ ions suggests the complicated CST test conditions, which can result in above-mentioned convoluted toluene emission behaviors. In an

A

effort to decipher this non-linear behavior, further studies will be done as follow-up tasks; the factors that contribute to propene or toluene adsorption/desorption and their conversion to other molecules will be elucidated by feeding each single component under both dry and wet conditions and by changing the amounts and locations of Cu species. Nevertheless, the Cu2+ ions were beneficial for helping toluene to be 26

later desorbed, as pronounced for Cu/M_30 (Fig. 10b2) that contained the largest amount of Cu2+ ions (Table 2). Regarding the evolution of CO2 and CO shown in Fig. 10b3, the above-mentioned CuO particles

SC RI PT

that can act as HC oxidation catalysts [18, 63] were apparently involved in the oxidation of propene/toluene and their derivatives. For Cu/L_100 and Cu/M_30, both CO2 and CO were generated, right after the emission amounts of propene and toluene were diminished (Fig. 10b1-b3). For Cu/H_100, however, although the propene and toluene were emitted earlier, CO2 and CO were generated later than those in Cu/L_100, possibly indicating that the oxidation performance was rather inferior to Cu/L_100.

U

The XRD analysis in Fig. S3 suggests that the amount of the ~5 nm sized CuO particles was smallest in

N

Cu/H_100, supporting the worse oxidation performance of Cu/H_100.

A

In summary, not only the large amount of B sites (relevant to ionic-exchanged Cu2+ ions) but

M

also the mesopores in SPP particles (relevant to the position of B sites and thus, Cu2+ ions) contributed to

D

improving propene and toluene adsorptions and achieving effective HC trap performance. Specifically, the large amount of mesopores enabled B sites to be located at the mesoporous and external surfaces in

TE

the H-form SPPs. Additionally, the Cu impregnation on these H-form SPPs allowed Cu2+ ions to be

EP

placed on the mesoporous and external surface. Thus, desorbed propene and toluene molecules could easily reach to CuO particles to be further oxidized to CO2 and CO, as proposed for the beneficial roles of

CC

the mesoporous materials in previous studies [17, 18]. Indeed, the mesopores introduced to the conventional microporous ZSM-5 apparently favored the formation of ~5 nm sized CuO particles, as

A

addressed in previous studies [17]. Although the difference in CO2 evolution was not pronounced in the three Cu-impregnated SPP particles, we would like to mention that Cu/M_30 showed the late desorption of both propene and toluene and accordingly, the late oxidation (~9 min, equivalent to 370 °C in Fig. 10b3) but with the highest HC conversion. 27

It appears that some portion of propene desorbed from the Cu-impregnated SPPs were converted into oligomers (corresponding to m/z = 56) at ~300 °C, except Cu/L_100 (Fig. 11b1). This inactive conversion in Cu/L100 could be ascribed to (1) the smallest amount of the B sites, which are known to

SC RI PT

accommodate propene oligomerization [70], and/or (2) the lowest desorption temperature, where a large portion of propene were already desorbed and released. The MS signals related to toluene disproportionation (m/z = 77 and 106) (Fig. 11b2-b3) evolved at the identical position where toluene was desorbed (Fig. 10b2). As the toluene desorption was stronger in increasing order of Cu/H_100, Cu/L_100, and Cu/M_30 (Fig. 10b2), the MS signals due to toluene disproportionation followed the same order. In

U

addition to the MS signals due to the toluene disproportionation, some new, but unclear peaks at the

N

elapse time of ~9-11 min (equivalent to ~320-370 °C) were observed for both Cu/H_100 and Cu/M_30. A

A

comparison with toluene desorption profile (Fig. 10b2) suggests that these additional MS signals resulted from the unwanted side reactions between toluene molecules. At this point, we would like to emphasize

M

that Cu-impregnated SPPs (especially, Cu/M_30) served as a good HC trap, as they can adsorb both

D

propene and toluene in the presence of steam. Although the oligomers (m/z = 56) and toluene

TE

disproportionation products (m/z = 77 and 106) would not be well-treated by TWCs, these unknown hydrocarbons were generated at high temperatures, where most TWCs are sufficiently thermally activated.

EP

Further temperature increase allowed for conversion to the desired CO2 and CO with the help of the CuO nanoparticles [18, 63] in Cu-impregnated SPPs (Figs. 10b3 and S13). To sum up these performances,

CC

Cu/M_30, which had the largest Cu2+ ions, could be used as a superior adsorbent for the HC trap among the three Cu-impregnated SPPs tested. In addition, CuO particles, mainly in the size of ~5 nm, in

A

Cu/M_30 (Figs. 7-8), will be supplemental to TWCs for the full oxidation of hydrocarbons. In order to further comprehend an effect of the hierarchal structure in SPP zeolites on the CST

performance, we also employed commercial, microporous ZSM-5 zeolites with and without impregnated Cu (C_75, C_140, Cu/C_75, and Cu/C_140). The resulting CST performances are shown in Figs. S1128

S12. Similar to the CST performance of Cu-impregnated SPPs (Fig. 10b1-b2), Cu2+ ions in Cu/C_75 and Cu/C_140 favored adsorbing propene and toluene, and concomitantly, retarded the desorption of propene and toluene. A larger amount of Cu2+ ions in Cu/C_75 resulted in much higher CST performances, as

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compared to Cu/M_30. In addition, it was noted that (1) Cu/C_140 had an amount of Cu2+ ions comparable to that in Cu/L_100 but less than that in Cu/H_100 (Tables 2 and S5), (2) Cu2+ ions in Cu/C_140 were mainly located in the micropores, and (3) Cu/C_140 showed a CST performance, which was comparable to that of Cu/H_100 and better than that of Cu/L_100. From these, it could be concluded that (1) a larger amount of Cu2+ ions enhanced the adsorption of propene and toluene and (2) Cu2+ ions at

U

the micropores contributed more effectively to improving the CST performance, especially via the

N

preferred adsorption of propene.

A

It appears that these underscored the importance in securing a large amount of Cu2+ ions just by

M

using a microporous ZSM-5, though such conclusions were not expected from the previous study [17], which emphasized the critical role of mesopores in a hierarchically structured ZSM-5 as an effective

D

support for CuO nanoparticles. However, in the light of a hydrothermal stability (e.g., treatment at 800 °C

TE

for 24 h in the presence of 10 vol% H2O vapor), the monotonic decrease of the Si/Al ratio in a microporous ZSM-5 would not be a sound approach. Instead, we recognized that the CST performances

EP

of the Cu-impregnated SPPs (Fig. 10b1-b3) and commercial ZSM-5 (Fig. S11b1-b3) were strongly convoluted functions of Si/Al ratios (relevant to Cu2+ ions) and mesoporosities (relevant to the location of

CC

Cu2+ ions and/or accessibility to and dispersion of CuO nanoparticles) of a zeolite support. Therefore, for securing long-term high CST performances, it is highly expected to require an optimized chemical

A

composition (relevant to Si/Al ratios and thus, Cu2+ ions) and/or an appropriate, hierarchical structure (relevant to mesopores and thus, CuO dispersion).

29

3.2.4. Hydrothermal stability of Cu-impregnated SPPs Fig. 12 shows the durability of Cu-impregnated SPPs as the HC trap through the three consecutive performance tests and their exposure to at a high temperature for hydrothermal treatment (in

SC RI PT

the presence of 10 vol% steam at 800 °C for 24 h that simulates a driving at more than 100,000 miles [71]). To the best of our knowledge, the hydrothermal treatment condition adopted in this study was extremely harsh compared to those employed in previous studies [55, 58, 72]. It was found that during the consecutive cycle tests (Fig. 12a1-c1), Cu/H_100 and Cu/M_30 showed similar propene and toluene emission profiles. However, for Cu/L_100, propene and toluene were emitted earlier in the third cycle.

U

According to the side product hydrocarbon profiles (Fig. S14), propene and toluene were more converted

N

to other hydrocarbons in the third cycle. Concomitantly, the CO2 and CO evolution times were delayed

A

less than 1 min and the corresponding CO2/CO production amounts were decreased. Despite gradual

M

performance degradation along the consecutive tests, all three Cu-impregnated SPPs showed the marked

D

ability as the HC trap.

In addition, all Cu-impregnated SPP HTs lost their original adsorption ability for both propene

TE

and toluene; propene and toluene were directly slipped to the exit stream without being captured. The

EP

emission amount of propene and toluene started to decrease at ~250-300 °C and eventually reached to zero at ~450-530 °C. Among the three Cu-impregnated SPP HTs, Cu/H_100 showed slightly better

CC

oxidation performances; initiation at ~250 °C and full oxidation at ~450 °C. The oxidation temperatures of all three Cu-impregnated SPP HTs moved to higher temperatures compared to those of the fresh

A

samples (Fig. 12a2-c2) and these oxidation temperatures became comparable to that of bulk CuO [73]. Nevertheless, the evolution amount of CO2 and CO revealed that HCs were almost converted to CO2 or CO, as supported by analyzing the corresponding emission profile (Fig. 12a2-c2) and TGA result (Fig. S15). TEM images (Fig. S16) and XRD patterns (Fig. S17) of the three Cu-impregnated SPP HT 30

confirmed the destruction of the MFI zeolite structure and the original CuO particles were well preserved. After the severe hydrothermal treatment, the MFI zeolite structure was transformed into a different structure, which was likely α-cristobalite (Fig. S17) [74]. Among the tested three samples, Cu/H_100 still

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had some degree of the MFI zeolite structure, while the other two were completely transformed to αcristobalite. Considering the formation of α-cristobalite is highly promoted by the alkali metals such as Na species [74], the phase transition of the SPP particles in the Cu-impregnated SPP HTs could be ascribed to a trace amount of residual Na species. Initially, we predicted that the SPP particle with the sharpest lamellae in Cu/H_100 would be most vulnerable to the phase-transformation and/or structural

U

degradation/damage, if allowed. Although we confirmed the negligible amount of Na species in all three

N

Cu-impregnated SPP particles (Table 1), it appears that Cu/H_100 contained the smallest amount of Na

A

species. This can be attributed to the effective ionic-exchange to the H-form SPP in H_100 due to its highest mesoporosity and thus, accessibility toward the Na species. Therefore, the resulting H_100 in

TE

4. Conclusions

D

M

Cu/H_100 seemed to resist its phase transition to α-cristobalite.

EP

SPP particles with various Si/Al ratios (nominal values of 30, 50, and 100) and mesoporosities have been synthesized. In particular, a simple removal of the ethanol and water content from the synthesis

CC

sol was a key to achieving the reliable formation of self-pillaring of MFI nanosheets or lamellae and thus, corresponding mesoporosity. In addition, we could synthesize the SPP with the Si/Al ratio as low as ~23

A

with a modest mesoporosity. It turns out that the degree of mesopores was increased with the increased Si/Al ratio, given the same sol composition, except for 30_E1_W1.0 that was rather amorphous. Three representative Na-form SPPs with different Si/Al ratios and mesoporosities were further ion-exchanged into H-form counterparts and further used as supports for Cu impregnation. It was found 31

that the CST performances of H-form SPPs themselves were insufficient to be used as effective HC traps, since they could not capture propene or toluene and even oxidize them at the high temperature of ~600 °C. However, a Cu-impregnated SPP, especially SPP with the Si/Al ratio of ~22 and a modest mesoporosity

SC RI PT

(Cu/M_30), showed a marked CST performance. Both propene and toluene were adsorbed considerably at the initial temperature of 70 °C in the presence of 10 vol% steam and started desorption at higher temperatures of 90 °C and 190 °C for propene and toluene, respectively. Finally, a larger amount of propene/toluene/their derivative hydrocarbons was effectively oxidized. The high CST performance could be attributed to (1) the improved propene adsorption that resulted from the ionic-exchanged Cu2+ ions and

U

to (2) the oxidation ability of the ~5 nm sized CuO particles that were dispersed on the SPP surface.

N

Although the CST performance was well maintained up to the three consecutive tests, a hydrothermal

A

treatment with ~10 vol% steam at 800 °C for 24 h failed the original performance of all three Cuimpregnated SPPs. The original SPP structure was collapsed and phase-transformed into other phase. In

M

the near future, we would like to (1) elucidate the specific role of Cu-impregnated SPPs on the HC trap

A

CC

EP

TE

D

and oxidation abilities and (2) improve their hydrothermal stability to be endured at harsh conditions.

32

Acknowledgements This work was supported by the Super Ultra Low Energy and Emission Vehicle (SULEEV) Engineering Research Center (2016R1A5A1009592) and by the C1 Gas Refinery Program

SC RI PT

(2015M3D3A1A01064957) through National Research Foundation (NRF) of Korea. These grants were funded by the Korea government (Ministry of Science and ICT). In addition, this research was supported by Korea University Future Research Grant. This research was also supported by the KBSI project (E37300). A part of SEM characterization and all of TEM characterization were carried out at the Korea Basic Science Institute (KBSI). In particular, all authors thank Dr. Eunjoo Kim for her acquiring TEM

A

N

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images and KBSI for allowing the usage of their HTREM instruments.

M

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A

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36

(b)

(020) (002)

5

100_E0_W0.5

50_E0_W0.5

30_E0_W0.5

100_E0_W1.0

50_E0_W1.0

30_E0_W1.0

100_E1_W1.0

50_E1_W1.0

(051) /(501)

(101)

(101)

(c)

(400) (301) /(040) (202)

(503)

(002)(012)(301)(131)(202) (020) (400) /(040) (111)

10

15

30_E1_W1.0

(303)

(101)

(051)(501)(303)(133)

Simulated MFI

20

25

30

35

(002)(012)(301)(131)(202) (020) (400) /(040) (111)

40 5

10

15

2 θ (°)

SC RI PT

Normalized intensity

(a)

(101)

(051)(501)(303)(133)

Simulated MFI

20

25

30

2 θ (°)

35

40 5

(002)(012)(301)(131)(202) (020) (400) /(040) (111)

10

15

(051)(501)(303)(133)

Simulated MFI

20

25

30

35

2 θ (°)

Fig. 1. XRD patterns of (a) 100_Ex_Wy, (b) 50_Ex_Wy, and (c) 30_Ex_Wy (three pairs of (x, y) = (1, 1.0),

U

(0, 1.0), and (0, 0.5)). For comparison, a simulated XRD pattern of all-silica MFI type zeolite is added.

N

For better understanding, the hkl indices of the survived peaks were included in (a) for a SPP particle

A

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EP

TE

D

M

A

(100_E1_W1.0), which exhibited the highest crystallinity among the synthesized nine particles.

37

40

SC RI PT U N A M D TE

Fig. 2. SEM images of (a1)-(a3) 100_Ex_Wy, (b1)-(b3) 50_Ex_Wy, and (c1)-(c3) 30_Ex_Wy (three pairs

EP

of (x, y) = (1, 1.0), (0, 1.0), and (0, 0.5)). For better understanding the effect of a molar composition of the

CC

synthetic sol on the particle synthesis, here we addressed the main difference in each synthetic procedure. Hydrothermal reaction was conducted by preserving the amount of ethanol and water for E1_W1.0 series

A

in (a1)-(c1), and by removing the amount of ethanol via evaporation for E0_W1.0 series in (a2)-(c2), and by removing the amount of ethanol and half of the amount of water for E0_W0.5 series in (a3)-(c3). The scale bars above the images indicate 500 nm.

38

SC RI PT U N A M D TE EP CC

A

Fig. 3. TEM images of (a1)-(a3) 100_Ex_Wy, (b1)-(b3) 50_Ex_Wy, and (c1)-(c3) 30_Ex_Wy (three pairs of (x, y) = (1, 1.0), (0, 1.0), and (0, 0.5)). The scale bars above the images indicate 100 nm

39

0.8

(a)

0.8

(b)

100_E1_W1.0 50_E1_W1.0

100_E0_W1.0 50_E0_W1.0 30_E0_W1.0

(c)

0.6

0.6

0.4

0.4

0.4

0.2

0.2

0.2

0.0

0

2

4

6

8

10

0.0

0

2

Pore width (nm)

4

6

Pore width (nm)

100_E0_W0.5 50_E0_W0.5 30_E0_W0.5

0.6

SC RI PT

Differential pore volume dV/dlog(D) (cm3∙g-1)

0.8

8

10

0.0

0

2

4

6

8

Pore width (nm)

Fig. 4. BJH pore size distributions of Na-form SPP particles based on the adsorption branch of N2

A

CC

EP

TE

D

M

A

N

U

physisorption isotherms at 77 K.

40

10

(a)

(111) (002) * *

(b)

M_30

H_100

L_100

5

10

15

(101)

(051)(501)(303)(133)

Simulated MFI

20

25

30

40 5

10

15

Cu/M_30 * * Cu/H_100 * *

Cu/L_100 * *

(051)(501)(303)(133)

Simulated MFI

20

25

30

35

2 θ (°)

M

A

2 θ (°)

35

(002)(012)(301)(131)(202) (020) (400) /(040) (111)

U

(002)(012)(301)(131)(202) (020) (400) /(040) (111)

N

(101)

SC RI PT

Normalized intensity

Simulated CuO

Fig. 5. XRD patterns of (a) H-form and (b) Cu-impregnated SPP particles. The simulated XRD patterns of

D

MFI and CuO are included for comparison. Asterisks (*) are inserted to indicate the XRD peaks of CuO

A

CC

EP

TE

phases.

41

40

SC RI PT U N A

M

Fig. 6. SEM images of (a1)-(a3) H-form and (b1)-(b3) Cu-impregnated SPP particles; three types of Naform SPP particles (100_E1_W1.0, 100_E0_W0.5, and 30_E0_W0.5) were chosen among the nine

D

different samples shown in Fig. 2 and further, converted into H-form SPPs and used as Cu-impregnated

A

CC

EP

TE

supports. The scale bars above all the images indicate 500 nm.

42

SC RI PT U N A M D TE EP CC

Fig. 7. Low magnification TEM images of (a1)-(a3) H-form and (b1)-(b3) Cu-impregnated SPP particles.

A

In the case of the Cu-impregnated SPP particles, the high magnification images were also given in (c1)(c3); white and black arrows are included in order to point to the ~5 nm and ~20 nm sized CuO particles, respectively.

43

SC RI PT U N A M D TE EP CC

Fig. 8. HAADF-STEM images (left) and the elemental Cu (middle) and Al (right) mapping of (a)

A

Cu/L_100, (b) Cu/H_100, and (c) Cu/M_30. The inset STEM images were used to measure the displayed elemental mapping. In the case of (a) Cu/L_100 and (c) Cu/M_30, the yellow-boxed region in each inset image was used to obtain the magnified image. For (b) Cu/H_100, the magnified image was taken at a different site. Note that the magnification in each image was not identical and refer to the scale bars inserted in the all images. 44

0.8

(a)

0.8

(b)

L_100 Cu/L_100

(c)

H_100 Cu/H_100

0.6

0.6

0.6

0.4

0.4

0.4

0.2

0.2

0.0

0

2

4

6

8

10

0.0

SC RI PT

Differential pore volume dV/dlog(D) (cm3∙g-1)

0.8

0.2

0

Pore width (nm)

2

4

6

8

10

0.0

0

2

Pore width (nm)

Fig. 9. BJH pore size distributions of H-form and Cu-impregnated SPP particles based on the adsorption

A

CC

EP

TE

D

M

A

N

U

branch of N2 physisorption at 77 K.

4

Pore width

45

(a1) propene

600

800

(b1) propene

Temperature

600

400

400

Temperature

600

400 Cu/M_30

400 L_100 M_30

200

200 Inlet conc.

H_100

0 0

5

10

Cu/H_100

0

0 20

15

0

5

L_100 H_100 M_30

400

U

Inlet conc.

N

A

0

5

10

15

0 20

Time (min)

5000

3000

(a3) CO2, CO

M

2500 2000

Temperature

1000 L_100 H_100

500 M_30

400 3000 200

artifact

TE 0

5

10

Cu/M_30

600

Cu/H_100 Temperature

Cu/L_100

400

15

0 20

200

1000 0

0

5

10

15

0 20

Time (min)

EP

Time (min)

(b3) CO2 CO

4000

2000

D

1500

600

Temperature (°C)

Concentration (ppmv)

200

Cu/M_30

0

0 20

15

400

200

Inlet conc.

10

Temperature

Cu/H_100

200

0

600

Cu/L_100

600

400

200

0

800

Temperature

600

Time (min)

CO2, CO

0 20

15

(b2) toluene

600

5

10

Temperature (°C)

Concentration (ppmv)

1000

(a2) 800 toluene

0

Inlet conc.

Time (min)

1000

400

200

200 Cu/L_100

Time (min)

Toluene

600

800

SC RI PT

Concentration (ppmv)

Cu-impregnated SPP 1000

Temperature (°C)

Propene

H-form SPP

1000

CC

Fig. 10. CST performance test results of (a1)-(c1) H-form and (a2)-(c2) Cu-impregnated SPP particles with respect to the representative component in the exit gas stream; (a1)-(a2) propene, (b1)-(b2) toluene,

A

and (c1)-(c2) CO2 and CO. The black line in each graph indicates the temperature profile. The dashed lines in (a1)-(a2) and (b1)-(b2) indicate the inlet concentrations of propene (100 ppmv) and toluene (100 ppmv), respectively. Test condition; the continuous feed comprising 100 ppmv propene, 100 ppmv toluene, 10 vol% H2O, 1 vol% O2, 560 ppmv Ar, and He balance at a WHSV = 100,000 mL∙g-1∙h-1 with the ramp rate = 50 °C/min. 46

H-form SPP

200

Temperature

Temperature

30

400

100

400 Cu/H_100

20

50 0

0

5

10

10 0

0 20

15

0

250

(a2)

L_100 H_100 M_30

(a3)

0

EP

0

D

20

TE

Signal (a.u.)

Temperature

30

5

10

Time (min)

600 400

U

Cu/H_100

15

0 20

Additional peaks

50

0

0

5

10

15

200 0 20

Time (min) 50

(b3)

600

40 Temperature

30 20

200

Cu/M_30

Cu/L_100

Additional peaks

Cu/H_100

10 0

0

5

400

Cu/M_30

10

15

200

Temperature (°C)

40

10

0 20

15

M

50

L_100 H_100 M_30

400

N

10

600

Cu/L_100

100 200

50 5

0 20

Temperature

150

A

Signal (a.u.)

400

Time (min)

m/z= 106

(b2) 200

Temperature

150

0

15

Temperature (°C)

600

200

0

10

Time (min)

250

100

200

5

Time (min)

m/z = 77

Cu/M_30

Cu/L_100

200

L_100 M_30

600

40

SC RI PT

Signal (a.u.)

m/z = 56

(b1)

600

H_100

Temperature (°C)

(a1) 150

Cu-impregnated SPP

50

0 20

Time (min)

CC

Fig. 11. MS signal profiles of side products from (a1)-(b1) propene oligomerization and (a2)-(a3) and (b2)-(b3) toluene disproportionation during the HC trap test with (a) H-form and (b) Cu-impregnated SPP

A

particles.

47

500

400

300

300

Propene 200 Toluene CO2 100

0

5

10

0 20

15

500

1000

400 500

0

Propene Toluene CO2

0

Concentration (ppmv)

600 Propene Toluene CO2 CO

(c2) Cu/M_30 HT 600

300

400

200

0 30

0 40

700

Temperature

800

500 600

Propene Toluene CO2 CO

200

600

300

400

200

0 10

Time (min)

20

30

0 40

600 500

400

100 0

100 0 20

10

700 1000

Temperature

800

400

100 20

0

Time (min)

500

10

0

200

Propene Toluene CO2 CO

200

400 300 200 100

0 0

10

20

30

Temperature (°C)

600

800

0

Propene Toluene CO2

(b2) Cu/H_100 HT 700 1000

Temperature

200

300

500

Time (min)

(a2) Cu/L_100 HT

400

400

100 0 20

10

Time (min)

1000

200

500

1000

SC RI PT

500

1000

700 Temperature 600

Temperature 600

Temperature (°C)

Concentration (ppmv)

700 1500

700 1500 Temperature 600

0

(c1) Cu/M_30 cycle

(b1) Cu/H_100 cycle

(a1) Cu/L_100 cycle 1500

0 40

Time (min)

U

Time (min)

N

Fig. 12. Stability tests on Cu-impregnated SPPs in CST; (a1)-(c1) cyclic tests and (a2)-(c2) test on HT

A

samples. In (a1)-(c1), straight and dotted curves represent the first cycle and third cycle results,

A

CC

EP

TE

D

M

respectively.

48

Table 1. Elemental analyses obtained from EDX of H-form and Cu-impregnated SPP particles.

Na/Ala

L_100

60.9 ± 27.7

0.1 ± 0.2

H_100

82.0 ± 20.6

0.2 ± 0.2

M_30

23.8 ± 2.2

0.0 ± 0.0

Cu/L_100

155 ± 51.2

0.1 ± 0.2

Cu/H_100

59.3 ± 10.6

0.1 ± 0.1

Cu/M_30

22.0 ± 2.8

0.0 ± 0.0

Cu wt%b

0.1 ± 0.2

-

0.2 ± 0.2

-

0.1 ± 0.1

-

3.3 ± 0.9

3.3

2.7 ± 0.4

3.5

5.6 ± 1.1

3.5

EDX data were measured at 10 individual particles in each sample to obtain the average and standard

N

deviation values.

Cu wt% data for Cu-impregnated SPPs were measured from the SEM/EDX mapping shown in Fig. S4

A

CC

EP

TE

D

M

A

b

Cu wt%a

SC RI PT

Si/Ala

U

a

Sample

49

Table 2. Pore structures and acid site titration results of H-form and Cu-impregnated SPP particles.

SBET (m2/g)

Smicro (m2/g)a

Smeso+ext (m2/g)a

Vmicro (cm3/g)a

V2-10 (cm3/g)b

L_100

393 ± 0.3

267

126

0.105

H_100

574 ± 1.2

162

412

M_30

501 ± 0.2

164

Cu/L_100

382 ± 0.1

Cu/H_100 Cu/M_30 a

Total

Externalc

0.027

45

40

19

0.082

0.244

109

100

38

337

0.068

0.139

191

121

31

266

116

0.105

0.043

21

8

145

488 ± 0.2

145

343

0.062

0.206

74

58

173

411 ± 0.2

172

238

0.071

0.103

122

99

338

Smeso+ext and Vmicro were obtained by using a modified t-plot method [35], and Smicro was indirectly

U

obtained by using the following equation; Smicro = SBET – Smeso+ext.

V2-10 was calculated from BJH pore size distribution (obtained from the adsorption branch) in the range

N

b

of 2-10 nm.

A

External B sites include the B sites located at both the external and mesoporous surface areas.

A

CC

EP

TE

D

M

c

L site (μmol/g)

SC RI PT

Sample

B site (μmol/g)

50