halloysite as an advanced three-way catalyst

Science of the Total Environment 707 (2020) 136137

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Sintering-resistant, highly thermally stable and well-dispersed Pd@CeO2/ halloysite as an advanced three-way catalyst Wei-Jing Li, Ming-Yen Wey ⁎ Department of Environmental Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• HNT promoted Pd@6CeO2 dispersion and enhanced high thermal stability. • Pd@6CeO2/H with optimal shell thickness to protect Pd from sintering and migration. • The negatively charged of HNT can attract the Pd@6CeO2 by electrostatic attraction. • Well-dispersed Pd@6CeO2/H-12 facilitated gas conversion and decreased T50.

a r t i c l e

i n f o

Article history: Received 1 October 2019 Received in revised form 25 November 2019 Accepted 14 December 2019 Available online 16 December 2019 Editor: Daniel CW Tsang Keywords: Thermal stability Particle dispersion Three-way catalyst Halloysite Pd@CeO2

a b s t r a c t The high thermal stability of halloysite (H)-supported core-shell Pd@CeO2 endowed it with promising catalytic performance and superior sintering resistance as a three-way catalyst. In this work, the synthesis of Pd@CeO2 nanoparticles with various shell thicknesses was performed, and the properties of the shell and support were examined. From the results, the Pd@6CeO2/H catalyst (Ce/Pd = 6) without any pretreatment or activation was achieved with a well-dispersed and optimal shell thickness of Pd@6CeO2 nanoparticles to inhibit sintering and aggregation via electrostatic attractions with halloysite. Moreover, the halloysite support imparted thermal stability for enhanced catalytic stability under long-term and high-temperature reaction conditions compared with Pd@6CZ/H (cerium-zirconium shell) and Pd@6CeO2/Al2O3 catalysts. To further ascertain the electronic effect on halloysite, Pd@6CeO2/H-12 (halloysite solution at pH = 12) was prepared. The results showed that Pd@6CeO2/H12 enhanced the catalytic activity and decreased the light-off temperature compared with the other studied catalysts, and these results were attributed to the high content of Ce3+ and oxygen vacancies and the strong interaction between Pd@6CeO2 and halloysite, making it a promising three-way catalyst. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Palladium (Pd) is the primary active component in most automotive three-way catalysts (TWCs) because it is the least expensive noble metal that is commonly loaded on alumina oxide to convert NOx, CO and HC pollutants to nontoxic gases (Chen et al., 2015; Li et al., 2018). However, Pd-based catalysts are prone to self-poisoning by ⁎ Corresponding author. E-mail address: [email protected] (M.-Y. Wey).

https://doi.org/10.1016/j.scitotenv.2019.136137 0048-9697/© 2019 Elsevier B.V. All rights reserved.

hydrocarbons, resulting in low NOx reduction ability compared to platinum and rhodium catalysts (Wang et al., 2015a). Moreover, TWCs are typically placed in a close-coupled configuration and exposed under high-temperature exhaust (higher than 1000 °C) during the warming up of engines (Chen et al., 2015; Sun et al., 2018). Such harsh conditions lead to metal active site sintering and support collapse, resulting in deactivation. To meet increasingly stringent emission control regulations worldwide, developing TWCs with high catalytic efficiency, low cost, low light-off temperature, and excellent thermal resistance is significant research.

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Alumina oxide material is a conventional support for three-way catalysts, and it has a large specific surface area that favors the dispersion of the active site (Wang et al., 2015a). Moreover, numerous reports have indicated that ceria could promote a Pd-only three-way catalyst with high catalytic efficiency due to the electron transfer between Ce3+ and Ce4+, which provides excellent oxygen storage capacity (OSC) (Hu et al., 2016; Song et al., 2015; Spezzati et al., 2019; Uzunoglu et al., 2017). On the other hand, the surface acidic sites of various pretreated supports also affect the catalytic performance (Gélin and Primet, 2002; Yazawa et al., 1999). Wang et al. demonstrated that the enhancement of three-way reaction of Pd/CZS catalyst can be attributed to the improvement of acid strength of support induced by Sr doping (Wang et al., 2008). Han et al. indicated that the introduction of Zr and Al into ceria lattice improves the oxygen mobility for oxygen storage capacity reaction (Han et al., 2010). According to the literature (Lan et al., 2015b; Zhao et al., 2010), ZrO2 incorporated with Al2O3 and CeO2 to form CZA composites could enhance the stability and mechanical properties of catalysts (Yan et al., 2019). However, especially under high temperatures, the thermal stability of CZA is limited, and the precious metal suffers from sintering and aggregation (Lan et al., 2018; Li et al., 2018; Yan et al., 2019; Zhao et al., 2010), resulting in the loss of the catalytic active site and even severe catalytic degradation (Cargnello et al., 2012; Wang et al., 2015b). Hence, to solve the problem of metal sintering, the metal as a core was protected by a shell material, forming a core@shell structure that has been identified as the most efficient way (Qi et al., 2012; Wang et al., 2015b; Zhang and Xu, 2013). A variety of metal@shell nanostructures have been successfully synthesized, such as Pd@CeO2 (Cai et al., 2018; Feng et al., 2017; Seo et al., 2017; Zhang and Xu, 2013), Pd@ZrO2 (Chen et al., 2014), Pd@SiO2 (Liu et al., 2015; Pi et al., 2016) and Au@CeO2 (Qi et al., 2012). Among them, numerous studies have been reported that on CeO2 as shell in core-shell structure not only can prevent Pd from sintering but also inhibit the growth of Pd particle (Li et al., 2018; Wang et al., 2015b; Wang et al., 2016). After densely coating with CeO2 shell, the Pd core can be protected physically to prevent mass transformation during both synthesis and long-term catalytic in high-temperature environment (Wang et al., 2015b; Zhang and Xu, 2013). In addition, the CeO2 components have excellent oxygen storage capacity and ability to generate strong synergistic effects associated with Pd metal (Wei et al., 2018; Song et al., 2015; Uzunoglu et al., 2017). On the other hand, to improve the problem of low thermal stability of conventional support for TWCs, the support material with superior thermal stability is referred to as a sintering barrier that incorporates a core@shell catalyst, which has received gradually attention recently (Cargnello et al., 2012; Li et al., 2018; Masias et al., 2015; Ozawa et al., 2017; Wang et al., 2015b). However, the hydrophobic alkyl covering of Pd@CeO2 nanoparticles with slightly positively charged particles experiences resistance from the hydrophilic surface of the support, such as the most commonly used Al2O3 material. Therefore, the low dispersion of Pd@CeO2 nanoparticles may result in their aggregation during the three-way reaction. A natural clay halloysite nanotube (HNT) is composed of 10–15 rolls of aluminosilicate sheets, and the inner void and external diameter are 15–30 nm and 50–100 nm, respectively; these HNTs have been considered eco-friendly, inexpensive and highly thermally stable nanomaterials (Jin et al., 2014; Kamble et al., 2012; Massaro et al., 2017). In particular, the unique HNT has an outer surface that is related to the negatively charged SiO2, while the inner cylinder core is similar to the positively charged Al2O3, suggesting that it attracts metal cations (Zhang et al., 2016) or positively charged nanoparticles to adhere to the HNT. Therefore, numerous studies have taken advantage of the electrochemical properties of HNT to act as an adsorbent in water treatment (Kamble et al., 2012; Pi et al., 2016; Shu et al., 2016). In terms of the chemical structures of HNT, Si-OH and Si-O are mainly exposed on the exterior, while Al-OH is present in the interior; such abundant surface groups are promising active sites for anchoring oxides to benefit heterogeneous catalysis (Kang et al., 2019). Moreover, the dealumination and

desilication of HNT material would occur under acid or alkaline treatment, which accompanied with the change of surface acid sites and electrochemical properties of HNT (Yuan et al., 2015). The pH of a solution can influence the overall charge of HNTs, indicated by changes in ξpotential. Especially in basic solution, HNTs were well-dispersed and each HNT end was separated from other HNTs due to van der Waals, allowing the opening access of the HNT inner pores (Joo et al., 2013; Yuan et al., 2015). Currently, there is a growing interest in the thermal stability of HNT on air pollutant purification. Zhou et al. and Ouyang et al. demonstrated that HNT could maintain its tubular structure at a high temperature of 1000 °C; moreover, the distinct structure acted as a tunnel for gas transportation and shortened the diffusion path of O atoms (Ouyang et al., 2017; Zhou et al., 2016). Sanchez-Ballester et al. reported that the Cu-Ni@halloysite catalyst showed a higher performance for CO oxidation than Cu-Ni/halloysite due to the metal active sites immobilized within the ligand-functionalized halloysite, thus resisting metal sintering (Sanchez-Ballester et al., 2015). Li and Wey fabricated well-dispersed Pd nanoparticles that were intercalated into functionalized halloysite, and this core-shell design of halloysite served as a sintering barrier that inhibited Pd sintering in three-way catalysis (Li and Wey, 2019). Indeed, halloysite with high thermal stability is a potential support for high-temperature reactions. Surprisingly, halloysite-supported core@shell nanoparticles have never been reported as three-way catalysts. In this work, we are inspired to synthesize halloysite-supported core-shell nanoparticles with an optimal shell thickness and shell properties by an electrostatic attraction-induced deposition method. CeO2 plays as promoter in Pd-based TWCs, which have excellent redox behavior and high oxygen vacancy concentration. However, CeO2-based composite suffers from sintering and aggregation after high temperature treatment, resulting in the loss of active oxygen sites and the deactivation. Thus, it is necessary to develop a core-shell structure with strong interaction between Pd and CeO2 to prevent metal nanoparticles from sintering. Meanwhile, the advantages of halloysite characterization including its high thermal stability and its electrochemical property to increase the particle dispersion and promote the interaction with core-shell nanoparticles, resulting in high catalytic activity and high thermal stability of this advanced three-way catalyst for the elimination of exhaust gas. The morphology, textural and structural properties, surface chemical state and redox ability, crystalline phase, and element analysis of the as-prepared catalysts were characterized by transmission electron microscopy (TEM), Brunauer-Emmett-Teller instrument (BET), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma optical emission spectrometer (ICP-OES), and zeta potential measurements. 2. Experimental section 2.1. Synthesis of core@shell nanoparticles First, the different shell thicknesses of Pd@mCeO2 (m for the molar ratio of Ce/Pd = 3, 6 and 9) were fabricated by adjusting the amount of Ce(NO3)3·6H2O (Acros, 99.5%) to obtain the optimized shell thickness. The preparation method of Pd@mCeO2 was described and modified in the literature (Wang et al., 2015b; Wang et al., 2016; Zhang et al., 2011). The Pd@mCeO2 were synthesized as follows: 1.5 g of KBr (Sigma-Aldrich) was dissolved into 100 ml of deionized water and heated at 60 °C for 15 min. Then, 1% Pd(NO3)2 solution (Alfa Aesar, Pd (NO3)2·xH2O, 39%) and different amounts of Ce(NO3)3·6H2O were added, followed by dropping 50 ml 0.005 M ammonia. The mixture was stirred at 60 °C for 1.5 h. Subsequently, the Pd@mCeO2 products were washed with deionized water and collected by centrifugation and then dried at 60 °C overnight. Another type of shell containing cerium-zirconium oxides (with a Ce:Zr molar ratio of 1:1) was synthesized by the above process and named Pd@mCZ nanoparticles. The various shell thicknesses and types of shells with the Pd@mCeO2 and Pd@

W.-J. Li, M.-Y. Wey / Science of the Total Environment 707 (2020) 136137

3

0.32 nm

20 nm

5 nm

CeO2 (111)

Fig. 1. TEM images of (a) Pd@3CeO2, (b) Pd@6CeO2, and (c) Pd@9CeO2. The (d) particle distribution of Pd@6CeO2 nanoparticles and its (e) FE-TEM images with (f–h) mapping analysis.

mCZ nanoparticles were further supported on halloysite to examine the catalytic activity. 2.2. Core@shell nanoparticles supported on halloysite Halloysite powder (Sigma-Aldrich, Al2Si2O5·2H2O) was mixed with 100 ml of deionized water or basic aqueous solution (pH-adjusted by ammonia solution), and the mixture solution was named H and H-p (p = 10 and 12 to represent pH = 10 and 12, respectively). The as-prepared

halloysite solution was heated at 80 °C in an oil bath and stirred for 2 h. Simultaneously, a given amount of Pd@mCeO2 nanoparticles was dispersed in 50 ml deionized water under ultrasonication. Afterward, the Pd@mCeO2 colloid solution was added dropwise into the as-prepared halloysite solution and then heated at 80 °C for 2 h. The mixture was washed and centrifuged with deionized water and then dried at 60 °C overnight. Finally, the sample was calcined at 600 °C for 3 h and formed Pd@mCeO2/H-p catalysts. The actual loading content of 0.88–1.06 wt% Pd was analyzed by ICP-OES, which is close to theoretical loading 1 wt%

Fig. 2. TEM images of fresh (top) and spent (bottom) catalysts: (a, d) Pd@3CeO2/H, (b, e) Pd@6CeO2/H, and (c, f) Pd@9CeO2/H.

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Pd. On the other hand, the Pd@CZ nanoparticles were supported on halloysite as well via a similar process as mentioned above. 2.3. Pd@mCeO2 nanoparticles supported on Al2O3 The Pd@mCeO2/Al2O3 catalyst was fabricated and compared with Pd@mCeO2/H for evaluating the properties of the support. The synthesis was similar to the above process, except this synthesis used Al2O3 (Alfa Aesar, S.A. 90 m2/g) instead of halloysite. 2.4. Characterization of catalysts TEM images were obtained on a JEOL JEM-1200CXII, and the morphology and particle size of the catalyst were analyzed. XRD was performed on a BRUKER D8-SSS with a Cu Kα radiation (λ = 0.154 nm) source and a scattering angle of 2θ = 5~80° at a rate of 2°/min to identify the crystalline phases of the catalysts. XPS patterns were obtained on a

PHI-5000 with Al Kα radiation (hυ = 1486.6 eV) and were used to analyze the chemical composition and the chemical state on the surface of the catalysts. C 1s was used as the reference to calibrate the binding energy of 248.8 eV. N2 adsorption-desorption was carried out at 77 K on a 201-AEL PMI instrument and calculated by BET method to measure the texture properties of the catalyst. The element analysis was analyzed by ICP-OES (OPTIMA 2000DV). A zeta potential analyzer (Malvern, Zetasizer Nano ZS) was used to measure the surface zeta potential of samples at 25 °C. In detail, 5 mg of the sample was dispersed and ultrasonicated in 15 ml of deionized water for 30 min to ensure a homogeneous dispersion. The average value of 20 measurements was reported here.

2.5. Catalytic activity tests The evaluation of the three-way catalytic activity was conducted by a fixed-bed continuous flow reactor. The catalyst (200 mg) was placed

CeO2 shell

Pd core

Fig. 3. TEM images of fresh (left) and spent (right) catalysts: (a, b) Pd@6CeO2/Al2O3, (c, d) Pd@6CZ/H, (e, f) Pd@6CeO2/H-10, and (g, h) Pd@6CeO2/H-12.

W.-J. Li, M.-Y. Wey / Science of the Total Environment 707 (2020) 136137

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Al2Si2O5(OH)4

Al2Si2O5(OH)4

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Pd@6CeO2/Al2O3

CeO2-ZrO2

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Fig. 4. XRD pattern of the as-prepared catalysts with various (a) shell thicknesses and (b) type of shells and supports.

in a quartz tube with an inner diameter of 1 cm and a length of 50 cm. Further, the quartz tube was placed in a tubular electric furnace with an outer/inner diameter of 20/3 cm. The simulated exhaust gas contained NO (1000 ppm), CO (0.3%), C3H8 (400 ppm), O2 (0.2–0.3%) and nitrogen as the carrier gas, and the total flow rate was 500 ml/min with a gas hourly space velocity of 64,000 h−1. The air/ fuel ratio (λ) was defined as λ = {2[O2] + [NO]}/{10[C3H8] + [CO]}, and λ = 1 was applied in all the catalytic activity measurements. The catalyst was pretreated at 300 °C for 2 h under 10% H2 with N2 prior to the three-way catalytic reaction. The inlet and outlet gases were continuously analyzed by a gas chromatography flame ionization detector (GC-FID) and an online flue gas analyzer (PG-250, HORIBA). The NOx, CO, and C3H8 conversions were calculated by the following equations: NOx conversion ð%Þ ¼

CO conversion ð%Þ ¼

½NOin −½NOout  100%: ½NOin

½COin −½COout  100% ½COin

C3H8 conversion ð%Þ ¼

½C 3 H 8 in −½C 3 H 8 out  100% ½C 3 H8 in

3. Results and discussion 3.1. Characterization of the catalyst For the morphology of Pd@mCeO2 nanoparticles with different shell thicknesses, as shown in Fig. 1a–c, the TEM analysis revealed that the Pd@6CeO2 had a uniform shape and that Pd was fully covered by CeO2. According to the literature (An and Somorjai, 2012; Jin et al., 2011; Wang et al., 2015b; Zhang et al., 2011), KBr plays two roles in promoting the growth and self-assembly of Pd@CeO2 nanoparticles. Firstly, Br− can retard the reduction of Pd2+ via the formation of the more stable complex of [PdBr4]2− for controlling the size of Pd cores. Secondly, KBr greatly decreases the colloid stability so that the surface polarityweakened Pd and CeO2 nanoparticles occur spontaneously selfassemble into more stable and ordered structure. On the other hand, the increased alkalinity of the solution could accelerate the assemblage of CeO2 nanoparticles to give the Pd cores timely stronger protection, resulting in the smaller size of Pd@CeO2 nanoparticles (Wang et al., 2015b). It can be concluded that the structure of Pd@CeO2 nanoparticles is strongly dependent on the addition amount of KBr and ammonia. However, the irregular and aggregated structure of Pd@3CeO2 and Pd@9CeO2 nanoparticles may attribute to the ratio of metal precursor,

KBr and ammonia are not appropriate for the formation of core-shell structure of Pd@CeO2. On the other hand, the particle size distribution, FE-TEM images and elemental mapping of Pd@6CeO2 are presented in Fig. 1d–h. As shown, the Pd is embedded in the center and surrounded with CeO2 particles aggregation to form a core-shell structure. Moreover, the core size and shell thickness of Pd@6CeO2 nanoparticles was approximately 10 nm and 13 nm, respectively. It also can be observed that the lattice fringe spacing of particles was 0.32 nm, which can be assigned to the (111) planes of the cubic fluorite phase of CeO2 (Lin et al., 2014; Spezzati et al., 2019), as shown in Fig. 1e. Furthermore, the fresh and spent Pd@mCeO2/H catalysts are presented in Fig. 2. As shown in Fig. 2a–b, the fresh Pd@3CeO2 and Pd@6CeO2 are well dispersed on the HNT support; however, the Pd@9CeO2 particles aggregated to a dense secondary particle owing to the grain growth and the presence of bulk CeO2 upon calcination. After a three-way catalytic reaction, the spent Pd@6CeO2/H catalyst could maintain a smaller particle size than Pd@3CeO2/H and Pd@9CeO2/H, as presented in Fig. 2d–f. Herein, it can be deduced that more uniform and complete core-shell structure of Pd@6CeO2/H exhibits high activity and sinteringresistance. To further understand the various properties of the shell and support, Fig. 3 shows the morphology of fresh and spent Pd@ 6CeO2/Al2O3, Pd@6CZ/H, Pd@6CeO2/H-10 and Pd@6CeO2/H-12 catalysts. The spent Pd@6CeO2/Al2O3 exhibited severe sintering (Fig. 3b), while slight aggregation occurred for the spent Pd@6CZ/H (Fig. 3d). In contrast, both the fresh Pd@6CeO2/H-10 and Pd@6CeO2/H-12 catalysts formed smaller particles with good dispersion (Fig. 3e and g), which was attributed to the increase in negatively charged halloysite with the increasing pH of the halloysite solution. This trend is beneficial for the catalytic reaction and for resistance to sintering, as shown in Fig. 3f and h. The XRD patterns were used to identify the crystalline phase of the as-prepared catalyst, while the dashed line and solid line represented the fresh and spent catalysts, respectively, as shown in Fig. 4. It was Table 1 Textural characteristics of the as-prepared catalysts.

Pd@3CeO2/H Pd@6CeO2/H Pd@9CeO2/H Pd@6CeO2/Al2O3 Pd@6CZ/H Pd@6CeO2/H-10 Pd@6CeO2/H-12

Surface area (m2/g)

Average pore diameter (nm)

Total pore volume (ml/g)

+

Ce3 /Ce (%)

Oβ/O (%)

37.25 38.05 35.44 71.35 42.26 34.44 29.95

18.80 20.01 19.38 12.92 17.93 20.42 20.82

0.17 0.19 0.17 0.23 0.19 0.18 0.16

19.67 22.47 18.75 19.58 21.55 23.95 24.42

48.60 49.96 48.16 49.48 49.72 51.49 53.95

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W.-J. Li, M.-Y. Wey / Science of the Total Environment 707 (2020) 136137 Table 2 Zeta potential value of as-prepared samples. Sample

Zeta potential (mV)

Pd@6CeO2 Neutral HNT H-10 H-12 Al2O3

16.4 −37.3 −47.5 −59.0 17.4

observed that all as-prepared catalysts had two peaks at 2θ = 20.8° and 26.5°, which were assigned to the Al2SiO2(OH)4 phase of halloysite (JCPDS 29-1487) (Jin et al., 2014; Lu et al., 2018), except for Al2O3 sample. Moreover, signals for the cubic fluorite structure of CeO2 were observed in all samples (JCPDS 34-0394). As shown in Fig. 4a, the peak intensity of the CeO2 phases increased with the shell thickness, which was attributed to the grain growth and the formation of bulk CeO2. The average crystallite size increases in the order of Pd@3CeO2/H (12.01 nm) b Pd@6CeO2/H (13.19 nm) b Pd@9CeO2/H (14.28 nm), which was calculated from the reflections of (111) using the Scherrer equation. Accordingly, Pd@9CeO2/H exhibits a high degree of the CeO2 crystalline phase compared with the other catalysts, suggesting that the bulk CeO2 particles are consistent with the results of the TEM analysis. It is noteworthy that no diffraction peaks were associated with the PdO and metallic Pd phase on the fresh or spent Pd@6CeO2/H and Pd@ 9CeO2/H catalysts, elucidating that Pd cores were still encapsulated by CeO2 shell nanoparticles under high-temperature calcination and reaction conditions. However, a small diffraction peak at 2θ = 42° of the Pd phase appears on the spent Pd@3CeO2/H, indicating that the shell

(a)

u'''

Ce 3d

v'''

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(b)

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915

u u'

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thickness is not enough to protect Pd cores from sintering. Fig. 4b shows the patterns of the fresh and spent Pd@6CeO2/Al2O3 and Pd@ 6CZ/H catalysts. The diffraction peak of Pd can be found owing to the low dispersion and interaction of core-shell nanoparticles and support, despite the catalysts having the optimal shell thickness. Consequently, the Pd@6CeO2/H with the optimal CeO2 shell is favorable for protecting the Pd cores from sintering, and the halloysite support plays an important role in hindering particle aggregation and enhancing the thermal resistance of the catalyst, suggesting the good thermal stability during the catalytic reaction. The textural properties of the as-prepared catalysts, including the specific surface area, average pore diameter, and total pore volume, are compiled in Table 1. The specific surface area decreases in the order of Pd@6CeO2/Al2O3 (71.35 m2/g) N Pd@6CZ/H (42.26 m2/ g) N Pd@6CeO2/H (38.05 m2/g) N Pd@3CeO2/H (37.25 m2/g) N Pd@ 9CeO2/H (35.44 m2/g) N Pd@6CeO2/H-10 (34.44 m2/g) N Pd@6CeO2/H12 (29.95 m2/g). This order shows that the excess shell thickness of Pd@9CeO2/H may result in a slightly decreasing surface area, while Al2O3 results in the highest surface area and the largest pore volume (0.23 ml/g) for Pd@6CeO2/Al2O3. Halloysite have two active sites for N2 adsorption-desorption, including in the inner-space and in the inter-space, wherein the inter-space surface area is more critical than the inner-space for the total surface area. According to the literature (Joo et al., 2013), the inter-space area obviously decreases when halloysite is dispersed in basic solution; thus, the resultant surface area decreases with increasing pH of the halloysite solution. Significantly, halloysite nanotubes disperse well, and the mesopores are opened in other halloysite nanotubes in basic solution, so the available average pore size increases for gas adsorption. The evaluation of the

Oβ

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900

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885

880

Binding energy (eV) O 1s

Oα

(d)

Oβ

O 1s

Oα

Pd@6CeO2/H-12

Pd@6CeO2/H

Intensity (a.u.)

Intensity (a.u.)

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Pd@6CZ/H

Pd@6CeO2/Al2O3

Pd@3CeO2/H

536

534

532

530 528 Binding energy (eV)

526

524

536

534

532

530 528 Binding energy (eV)

Fig. 5. XPS spectra of (a, b) Ce 3d and (c, d) O 1s of the as-prepared catalysts.

526

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C3H8 conversion (%)

Pd@6CeO2/H 80

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Temperature (°C) Fig. 6. The gas conversion curves of (a) NOx, (b) CO, and (c) C3H8 for the Pd@mCeO2/HNT catalysts.

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(a)

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three-way catalytic reaction (Section 3.2.2) showed that Pd@6CeO2/H12 had the best catalytic activity, even though the surface area was not the largest among the other catalysts. To fully understand the electrical property of the samples, the zeta potential of halloysite, Al2O3, and the Pd@6CeO2 was measured under the synthetic process, as shown in Table 2 and Fig. S1. It can be found that neutral halloysite with negatively charged (−37.3 mV), and the zeta potential value of halloysite decreases with the increasing pH of the halloysite solution, which is contributed to attracting positively charged Pd@6CeO2 particles (16.4 mV) during the synthesis process, resulting in well-dispersed Pd@6CeO2 exhibits strong interaction with the halloysite support. However, Al2O3 with a positive charge (17.4 mV) experiences electrostatic repulsion with the Pd@6CeO2 nanoparticles, it is reasonable to assume that a relatively weak interaction between Pd@6CeO2 and Al2O3. Consequently, these results demonstrated that halloysite has good interaction with Pd@6CeO2 nanoparticles even without any basic treatment, contributing to a welldispersed Pd@6CeO2/H catalyst. This phenomenon is consistent with the TEM results and the evaluation of catalytic performance. The chemical state and the atomic ratio of the surface elements for the as-prepared catalysts were measured by XPS analysis. First, attention should be paid to the effect of chemical state of Pd species on catalytic performance. As shown in Fig. S2, the Pd 3d spectra can be fitted into four peaks associated with two spin orbits, and the peaks within the range of 340.8–342.2 eV and 335.7–336.8 eV are assigned to Pd 3d3/2 and Pd 3d5/2, respectively (Sun et al., 2018; Yan et al., 2019; Zhao et al., 2010). It is also noticed that the total Pd 3d peak area decreases with the increasing Ce shell thickness, suggesting that Pd particles are encapsulated by the CeO2 and cannot be precisely detected in XPS depth profile. Thus, the fractions of Pd2+ species are roughly calculated and presented a decline tendency of Pd@6CeO2/H (55.4%) N Pd@ 3CeO2/H (54.5%) N Pd@9CeO2/H (35.9%). In addition, the obtained Ce 3d XPS spectra of the as-prepared catalysts are depicted in Fig. 5a and b. The spectra can be deconvoluted into eight peaks related to four different spin-orbit doublets. As shown in the Ce 3d spectra, the peaks marked as u and v are assigned to 3d3/2 and 3d5/2, respectively. Among them, the signals resulting from Ce4+ 3d3/2 were labeled as u, u″ and u‴, while the signals arising from Ce4+ 3d5/2 were marked as v, v″ and v‴ and the doublet u′ and v′ were assigned to Ce3+ 3d3/2 and Ce3+ 3d5/2. The fraction of Ce3+ in the total Ce species of as-prepared catalysts is calculated and listed in Table 1. Among the seven catalysts, Pd@6CeO2/H-12 showed the highest ratio of Ce3+/Ce, and the sequence of Ce3+/Ce decreased in the order of Pd@6CeO2/H-12 (24.42%) N Pd@ 6CeO2/H-10 (23.95%) N Pd@6CeO2/H (22.47%) N Pd@6CZ/H (21.55%) N Pd@3CeO2/H (19.67%) N Pd@6CeO2/Al2O3 (19.58%) N Pd@ 9CeO2/H (18.75%). It is noticed that Pd@3CeO2/H and Pd@9CeO2/H exhibit relatively low surface concentration of Ce3+ and Pd2+ may be caused by particles growth and formation bulk CeO2 after calcination process. Therefore, the suitable shell thickness of CeO2 can promote excellent redox properties. The high thermal stability of halloysite, which can act as a stabilizer of the CeO2 and CZ mixture (Ouyang et al., 2014; Zhou et al., 2016) and as a sintering barrier for the catalyst, enhanced the TWC performance. As demonstrated in the literature (Lan et al., 2015a; Lan et al., 2018; Yan et al., 2019), Ce3+ is related to the oxygen vacancies that can facilitate interactions between Pd and CeO2, which is of great importance to the redox property and catalytic activity. Fig. 5c and d depict the O 1s XPS spectra of the as-prepared catalysts. The peaks are asymmetric, indicating that at least two different O 1s chemical species exist in the catalysts. The peaks at low binding energies (529.8–530.1 eV) labeled as Oα can be assigned to lattice oxygen (O2− 2 ), while those at high binding energies (531.3–533.3 eV) marked as Oβ are − attributed to surface-adsorbed oxygen (O− 2 of O ) or oxygen vacancies (Hu et al., 2016; Lan et al., 2018); it is well known that Oβ is considered to be an active oxygen species due to its high mobility. The Oβ trend for the as-prepared catalysts is consistent with the results obtained from the Ce 3d XPS spectra, which precisely corresponds with the catalytic

performance; Pd@6CeO2/H-12 has the highest amount of Ce3+/Ce and Oβ, resulting in the promotion of the redox ability of the TWC reaction. 3.2. Catalytic activity 3.2.1. Effect of CeO2 shell thickness The three-way catalytic performance was examined, and the conversion of NOx, CO, and C3H8 for Pd@mCeO2/H catalysts is presented in Fig. 6. As shown in Fig. 6, it illustrates that Pd@6CeO2/H exhibits high thermal stability and catalytic activity, especially in the elimination of NOx, while Pd@3CeO2/H shows a lower performance for the NOx conversion at 400–600 °C, suggesting that the shell thickness is insufficient to resist Pd particle migration and sintering at high reaction temperatures. On the other hand, the Pd@9CeO2/H has poor gas conversion owing to the mass transfer limitations in bulk particles, and the halloysite tunnel may be plugged by the aggregation of large Pd@9CeO2 particles. The light-off temperature (T50) for NOx, CO, and C3H8 over the as-prepared catalysts are listed in Table 3. For the Pd@ 6CeO2/H catalyst, the T50 values of the NOx, CO and C3H8 conversions were 268, 185, and 295 °C, respectively, with superior catalytic performance compared with Pd@3CeO2/H and Pd@9CeO2/H. The Pd@6CeO2/ H catalysts have a high content of Ce3+ and adsorbed oxygen (Oβ), which can facilitate the redox reaction; thus, the catalytic activity result is consistent with the XPS results. Herein, it is illustrated that the Pd@ 6CeO2/H catalyst had an optimal shell thickness to resist sintering and that was favorable for three-way catalysis. 3.2.2. Effect of the cerium-zirconium shell and Al2O3 support To further study the various properties of the shell and support on the three-way reaction, the catalytic performance of Pd@6CeO2/H, Pd@6CZ/H and Pd@6CeO2/Al2O3 is shown in Fig. 7, and the data of T50 are compiled in Table 3. As shown in Fig. 7, the shell and support properties were substituted with cerium-zirconium oxides and Al2O3, respectively. The addition of zirconium oxides did not enhance the thermal stability and degrade the catalytic activity of Pd@6CZ/H, but Pd@6CeO2/Al2O3 showed the lowest T50 for CO conversion and the highest C3H8 conversion. Generally, CeO2-based catalysts can stabilize oxidized metal species via the interaction of surface oxygen; however, the interaction between PdOx and Ce-Zr components would be weakened through high-temperature conditions owing to the encapsulation of PdOx, resulting in a decline in the catalytic reaction (Lin et al., 2014; Wu et al., 2004). On the other hand, compared with halloysite, Al2O3 is a relatively weak acid for support materials, and it is prone to adsorbing more acidic gases, such as NOx and C3H8, thus exhibiting a higher C3H8 conversion under high-temperature conditions (Yazawa et al., 1999). Although Pd@6CeO2/Al2O3 shows the maximum C3H8 conversion under high-temperature reaction, Pd@6CeO2/H has the most stable catalytic performance and exhibits the lowest T50 for NOx and C3H8 conversion among the studied catalysts. Moreover, a thermal stability test of Pd@6CeO2/H, Pd@6CZ/H and Pd@6CeO2/Al2O3 is presented in Fig. 8. As shown in Fig. 8, the Pd@6CeO2/H catalyst exhibits the highest catalytic activity regardless of whether NOx or CO conversion occurs within the long-term reaction at high temperatures. On the other hand, Fig. S3 (Supporting Information) presents the effect of Table 3 The light-off temperature (T50) of the as-prepared catalysts. NOx

CO

C3H8

T50 (°C) Pd@3CeO2/H Pd@6CeO2/H Pd@9CeO2/H Pd@6CeO2/Al2O3 Pd@6CZ/H Pd@6CeO2/H-10 Pd@6CeO2/H-12

273 268 333 272 285 268 248

187 185 216 200 173 180 150

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Fig. 7. The gas conversion curves of (a) NOx, (b) CO, and (c) C3H8 on the catalysts with the same shell thickness but various shell types and supports.

250

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(a)

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Fig. 8. The thermal stability test: the gas conversion curves of (a) NOx and (b) CO of the catalysts with the same shell thickness but various shell types and supports. (Reaction conditions: NOx: 1000 ppm, CO: 3000 ppm, C3H8: 400 ppm, O2: 0.2–0.3% and N2 as the carrying gas at 400 °C).

Fig. 9. The conversion curves of (a) NOx, (b) CO and (c) C3H8 over Pd@6CeO2/H-p via a basic treatment.

halloysite support on Pd@6CeO2 nanoparticles. It shows that the CO conversion of Pd@6CeO2 is obviously lower than Pd@6CeO2/H, followed by the trend of CO conversion gradually decreases after 6-hour reaction due to the aggregation of Pd@6CeO2 nanoparticles. By contrast, Pd@ 6CeO2/H can keep high stability and redox ability during long-term reaction, attributed to the halloysite support enhances the thermal resistance and promotes well dispersion of Pd@6CeO2 nanoparticles with high redox property. 3.2.3. Modification of halloysite via a basic treatment To discuss the effect of a basic treatment on the halloysite support in further detail, Fig. 9 shows the catalytic activity of Pd@6CeO2/H-p catalyst with various pH of halloysite solution The catalytic activity remains stable at high temperatures, and the value of T50 distinctly decreases, especially for the Pd@6CeO2/H-12 catalyst. Accordingly, the decomposition of halloysite is initiated on the inner surface of the nanotubes, and the solubility of Si4+ is greater than that of Al3+ in an alkaline environment, leading to the formation of flat Al(OH)3 nanosheets (Kamble et al., 2012; Yuan et al., 2015) and an increase in the adsorption of acidic gases. Moreover, halloysite in a higher pH solution gives a significantly low zeta potential value (−59.0 mV), which contributes to a good dispersion of particles and a strong interaction with positively charged Pd@6CeO2 nanoparticles. As stated above, the chemical properties of halloysite exert a crucial influence on the catalytic performance. Therefore, the modification of halloysite via basic treatment can notably promote C3H8 conversion and decrease the overall light-off temperatures.

nanoparticles was synthesized by a simple electrostatic attraction-induced deposition method in which the Pd cores were encapsulated by an optimal thickness of CeO2 (Ce/Pd = 6), which is favorable for resisting sintering and aggregation. Moreover, compared with Pd@6CZ/H and Pd@6CeO 2 /Al 2 O 3 , the Pd@ 6CeO2/H catalyst had good catalytic stability and redox property, along with excellent thermal stability, attributed to the high content of Ce3+ and Oβ and the superior thermal stability associated with the halloysite support. Most importantly, when halloysite is submitted to basic treatment, a significant electrostatic attraction takes place. Halloysite in a high pH solution has a low zeta potential, and there is a strong interaction between Pd@6CeO2 nanoparticles and the halloysite. In addition, the increased dispersion of the Pd@6CeO2/H-12 can facilitate the adsorption of acidic gases, which consequently gives rise to enhanced gas conversion and the best catalytic performance. In summary, new insights were gained into the characterization of halloysite and its influence on the performance of supported Pd@6CeO2 core@shell nanoparticles, which are promising three-way catalysts for real-world applications.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

4. Conclusion

Appendix A. Supplementary data

Based on the analysis results presented above, a high thermalresistant halloysite support with well-dispersed Pd@6CeO 2

Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2019.136137.

W.-J. Li, M.-Y. Wey / Science of the Total Environment 707 (2020) 136137

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