Excellent catalytic activity and water resistance of UiO-66-supported highly dispersed Pd nanoparticles for toluene catalytic oxidation

Journal Pre-proof Excellent catalytic activity and water resistance of UiO-66-supported highly dispersed Pd nanoparticles for toluene catalytic oxidation Fukun Bi (Writing - original draft), Xiaodong Zhang (Writing - review and editing) (Conceptualization) (Funding acquisition), Jinfeng Chen (Data curation), Yang Yang (Visualization) (Investigation), Yuxin Wang (Software) (Validation) (Funding acquisition)

PII:

S0926-3373(20)30182-X

DOI:

https://doi.org/10.1016/j.apcatb.2020.118767

Reference:

APCATB 118767

To appear in:

Applied Catalysis B: Environmental

Received Date:

31 December 2019

Revised Date:

6 February 2020

Accepted Date:

14 February 2020

Please cite this article as: Bi F, Zhang X, Chen J, Yang Y, Wang Y, Excellent catalytic activity and water resistance of UiO-66-supported highly dispersed Pd nanoparticles for toluene catalytic oxidation, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118767

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Excellent catalytic activity and water resistance of UiO-66-supported highly dispersed Pd nanoparticles for toluene catalytic oxidation Fukun Bi a, Xiaodong Zhang a, *, Jinfeng Chen a, Yang Yang a, Yuxin Wang b a

School of Environment and Architecture, University of Shanghai for Science and Technology, Shanghai 200093, China Institute of Applied Biotechnology, Taizhou Vocation & Technical College, Taizhou

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Zhejiang 318000, China

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* To whom correspondence should be addressed. Tel. +86 15921267160, Fax. +86 021 55275979 E-mail address: [email protected] (X.D. Zhang) 1

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Graphical abstract:

 The

Pd-U-EG

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Highlights: displays

an

excellent

catalytic

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performance and water resistance.

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 Excellent activity is ascribed to highly dispersion Pd NPs and high Olat/Oads.

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 Excellent water resistance

is

demonstrated

by

toluene-TPSR and in-situ DRFTS.

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 The reaction mechanism of toluene combustion over

Pd-U-EG is investigated.

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ABSTACT The highly dispersed Pd nanoparticles supported UiO-66 catalysts were successfully prepared via ethylene glycol reduction method (Pd-U-EG). And their catalytic performances were evaluated by toluene degradation. A series of characterization methods were carried out to characterize the physicochemical properties of the

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samples. During the effect of high weight hourly space velocity, stability and reusability test, the catalytic activity of Pd-U-EG remains unchanged, which also

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indicated good catalytic performance. More importantly, water resistance test (10-20 vol.% water) indicated that Pd-U-EG had a great water resistance. The study of

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toluene-TPD, toluene-TPSR and in-situ DRIFTS at different temperatures under

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different conditions over Pd-U-EG indicated the role of H2O. The introduction of H2O at low temperature was conducive to the adsorption of toluene, but inhibited the

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degradation of toluene. Differently, the H2O presence at high temperature was favorable to toluene degradation. In addition, toluene degradation mechanism was

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also revealed.

Key words: catalytic oxidation, toluene, Pd-UiO-66 catalysts, toluene-TPD, in-situ DRIFTS

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1. Introduction Volatile organic compounds (VOCs), as a major air pollutant, discharged from transportation, industrial production and other activities of humankind, have brought a pernicious influence to human health and the environment [1-5]. To reduce the elimination of VOCs, many technologies have applied, including adsorption [6-8],

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photocatalysis [9, 10], plasma [11,12] and catalytic oxidation [13-17]. Among them, as one of the most effective methods, catalytic oxidation has been widely used due to

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its low energy consumption and no secondary pollution. Since the catalyst is the core

endurable catalysts [18-20].

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of catalytic oxidation, great efforts have been made to find efficient, economical and

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The catalysts are usually used for catalytic oxidation, include noble metal catalyst [21-24], transition metal catalysts [25-28] and zeolites [29]. Noble metal

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catalysts, especially Pd, Pt, etc., have been widely applied due to their distinct physical and chemical properties and remarkable catalytic activity. However, owing

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the high surface energy and mobility, the noble metal particles tend to aggregate

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during synthesis process of the sample or the oxidation reaction. Thus, it has been widely studied to load the noble metal onto a carrier to obtain a catalyst with good dispersibility [30-35]. As an emerging porous coordination polymer, metal organic frameworks (MOFs) have superior properties and have applications in adsorption [36], photocatalysis [37] 4

and catalysis [38] etc., compared with conventional supports such as zeolite [39], mesoporous silica [40], alumina [41] and porous carbon [42], etc. Among the numerous MOFs, UiO-66 is widely used as a support for precious metals because of its good thermal stability, good water and organic solvent resistance [43-45]. In the past few years, a great number of researches have been paid attention to the preparation of highly dispersion Pd-UiO-66 or Pd-UiO-66-NH2 with various methods,

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such as impregnation method [46], NaBH4-reduction [47], double-solvent [48] and etc. Jiang et al. prepared highly dispersion Pd@UiO-66 and Pd/UiO-66 for catalytic

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ethanol to synthesis n-butanol by double-solvent method and impregnation method, respectively [49]. It was found that Pd nanoparticles had a good catalytic effect on

Pd-UiO-66

and

Pd-UiO-66-NH2

with

good

dispersibility

via

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synthesized

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dehydrogenation of ethanol and hydrogenation of crotonaldehyde. Guan et al.

impregnation method for phenol hydrogenation [50]. The results showed that the

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hydrogenation reaction rate of Pd-UiO-66 was significantly higher than that of Pd-UiO-66-NH2. In a word, highly dispersed Pd nanoparticles are key to the synthesis

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of highly active catalysts, and the preparation methods have great influence on the

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dispersion of Pd nanoparticles. Therefore, it is important to find a method to prepare a catalyst with high dispersion Pd nanoparticles, high catalytic activity, water resistance and stability. The purpose of this study is to seek a facile synthesis method to prepare a catalyst with high activity, high water resistance, high stability and high dispersion of Pd nanoparticles. 5

In this work, three different reduction methods, including pre-impregnation, NaBH4-reduction and ethylene glycol-reduction methods, were used to prepare different Pd-UiO-66 catalysts. The effects of different synthetic methods on the structure and the catalytic activity for toluene oxidation of the catalyst were explored. And the toluene degradation mechanism and the effect of H2O were also explored by the toluene-TPD, toluene-TPSR and the in-situ DRIFTS. This is the first time that the

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Pd-UiO-66 was used for catalytic degradation of gaseous pollutants toluene and study the effect of H2O and degradation mechanism.

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2. Experimental 2.1 Chemicals

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Zirconium (IV) chloride (ZrCl4, 98%, Aladdin), Pd(OAc)2 (Pd, 47.4%, Aladdin),

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terephthalic acid (H2BDC, 99%), N, N-dimethylformamide (DMF, 99.5%), absolute ethanol (99.7%), acetic acid (36%) and ethylene glycol (EG, 99%) were bought from

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Sinopharm Chemical Reagent Co., Ltd. Sodium tetrahydroborate (NaBH4, 98%) was purchased from Shanghai Titan scientific Co., Ltd. All the chemicals were analytical

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grade and were used directly.

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2.2 Catalysts preparation 2.2.1 Synthesis of UiO-66 The UiO-66 was synthesized by a conventional solvothermal synthesis (Scheme

1(a)) [6]. In this method, 0.053 g ZrCl4 (0.227 mmol) and 0.034 g H2BDC (0.227 mmol) ultrasound dissolved in 5 mL DMF. After that, the obtained mixture was 6

transferred in a Teflon-lined autoclave and reacted in an oven at 120 °C for 24 h. After cooling down to ambient temperature, the white powder could be got by centrifugation and washing with ethanol for 3-4 times, and finally dried at 70 °C overnight in an oven. 2.2.2 Synthesis of Pd-UiO-66-H2 The Pd-UiO-66-H2 catalyst was prepared via a pre-impregnation method [22] by

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using Pd(OAc)2 as the precursors. The as-prepared UiO-66 support was impregnated

with an acetic solution of Pd(OAc)2, and dried at environment temperature for 12 h,

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then dried at 70 °C overnight in an oven. Finally, the catalyst named as Pd-U-H could be obtained by reduction in a H2 flow (30 mL/min) at 200 °C for 2 h. The theoretical

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2.2.3 Synthesis of Pd-UiO-66-NaBH4

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loadings of Pd nanoparticles (NPs) was 1.0 wt%.

Supported 1.0 wt% Pd-UiO-66-NaBH4 catalyst was prepared by using NaBH4 as

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reducing agent [51]. 1.0 g UiO-66 was dispersed in 50 mL deionized water. An acetic solution of Pd(OAc)2 was added to the above turbid liquid under energetic stirring and

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continued to stir for 24 h. After that, the NaBH4 (NaBH4/Pd = 10, molar ratio) [52]

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was added into the mixture under ice bath condition for 2 h to reduce Pd2+. The catalyst donated as Pd-U-NH could be acquired by centrifugation and washing with deionized water and ethanol for four times, and then dried at 70 °C for 12 h. Before characterization tests and catalytic experiments, the samples were purged in an Ar ambience at 100 °C for 1 h. 7

2.2.4 Synthesis of Pd-UiO-66-EG Pd-UiO-66-EG catalyst with 1.0 wt% Pd NPs was prepared by using EG as the reducing agent and solvent [53]. Usually, 1.0 g of UiO-66 was dispersed in 150 mL EG under magnetic stirring for 30 min. An acetic solution of Pd(OAc)2 was added dropwise into the UiO-66 suspension with energetic stirring. The mixed solution was subsequently heated under continuous magnetic stirring at 150 °C for 2 h. After

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cooling to room temperature, the catalyst which was abbreviated as Pd-U-EG can be get after washing thoroughly with deionized water and ethanol for three times and

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drying at 70 °C for 12 h. Before characterization tests and catalytic experiments, the

2.3 Catalysts characterization

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samples were purged in an Ar atmosphere at 100 °C for 1 h.

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The powder X-ray diffraction (XRD) patterns was detected on a Bruker D8 Advance X-ray diffractometer using a Cu Kα (40 kV, 40 mA) radiation

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monochromatic detector with a scan rate of 5°/min, scanning range of 10°~80°. The pore properties of the catalysts were determined by N2 adsorption-desorption at 77 K

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(liquid nitrogen temperature) on a Quantachrome autosorb-iQ-2MP apparatus. Before

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analysis, the samples were outgassed under vacuum at 105 °C for 12 h to purification. The Brunauer-Emmett-Teller (BET) model and the Langmuir model were utilized to calculate the specific surface area, the pore size distributions were generated by the Density-functional-theory (DFT) method. The scanning electron microscopy (SEM) images of the samples were taken by a Philips FEIXL-30 operated by using a 20 kV 8

accelerating voltage after gold deposition. The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) images were performed with a TECNAI G2 F20 instrument. To understand the chemical states, surface property and composition of the catalysts, the X-ray photoelectron spectroscopic (XPS) (THERMO ESCALAB 250XZ) analysis was utilized. The binding energies were calibrated internally by the carbon deposit C1s at 284.8 eV.

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UV-vis diffuse reflectance spectra were performed on a UV-vis spectrometer (UV-2600, Shimadzu, Japan) and the BaSO4 was used as the internal standard.

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Temperature programmed toluene desorption (toluene-TPD) and temperature programmed toluene surface reaction (toluene-TPSR) was taken on fixed bed

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microreactor. The catalysts (100 mg) were purged with Ar at 100 °C for 1 h, prior to

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the experiment. In the ambient temperature, toluene vapor and O2 or toluene vapor, O2 and water vapor were fed into the catalyst until adsorption saturation. Next, the

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adsorption saturated catalyst was purged for 1 h under an Ar atmosphere to remove weakly adsorbed toluene + O2 or toluene + O2 + H2O. Finally, in an argon flow

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temperature programming was started at a heating rate of 5 °C/min up to 500 °C. The

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above gas flow rate was 50 mL/min. A mass spectrometer (MS, SOPTOP, Shanghai) was used for on-line monitoring of exhausted gases. The signals at mass-to-charge (m/z) ratios of 44 (CO2) ， 72 (C4H4O4), 77 (C6H5CHO), 91 (C7H8) and 105 (C6H5COOH) were detected. Toluene-TPSR was conducted the same program as toluene-TPD. The gas used in temperature programming process was 20% O2/Ar flow 9

in 50 mL/min. The in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS, Nicolet iS50, Thermo Fisher, America) equipped with Harrick in-situ cell was used to explore the mechanism of toluene catalytic combustion. About 50 mg catalyst was added into the hold of diffuse reflection cell. The sample was first pre-treatment under an Ar atmosphere at 200 °C for 2 h, and then the sample was cooled to ambient

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temperature. The gas was switched to 20% O2/Ar, and DRIFTS of as-pretreated

sample was taken as background. Then, gaseous toluene was introduced by 20%

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O2/Ar for 1 h. Finally, the sample temperature rose to the target temperature and the signal was collected.

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2.4 Catalytic activity test

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Toluene catalytic degradation was determined in a fixed bed flow reactor at atmospheric pressure. 100 mg (20-40 mesh) sample was packed in a U-shaped quartz

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reaction tube (outside diameter 6 mm). Catalytic performance was determined at a total flow rate of 50 mL/min (weight hourly space velocity (WHSV) =30000 mL/g/h)

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and controlled by an independent thermal mass flow controller. An air stream

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containing 1000 ppm of toluene passed through the sample to estimate catalytic activity. Effect of water vapor on the catalytic activity was investigated by introducing 5 vol%, 10 vol% or 20 vol% H2O by the feed stream through a water saturator at certain temperature, and calculated by the following formula: ψ=φPv/P, where ψ was water vapor content, φ was relative humidity, Pv was the saturated 10

vapor pressure of water at a certain temperature and P was standard atmospheric pressure. On-line gas chromatograph (GC2060, Ruimin, Shanghai) equipped with a flam ionization detector (FID) was applied to determine the reactor inlet and outlet off-gas streams. To evaluate the catalytic activities of the catalysts, the temperature (T10%, T50% and T90%) consistent with toluene conversion of 10, 50 and 90% were used.

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Toluene conversion could be calculated by the following formula: X=(Cin - Cout) / Cin×100%

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where X was toluene conversion, Cin and Cout were toluene concentration in inlet and

3. Results and discussion

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3.1 Catalysts characterization

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outlet gas, respectively.

The crystallinity and phase purity of the as-prepared UiO-66 and Pd-U catalysts

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were identified by XRD technique. As shown in Fig. 1(a), the peak shape of UiO-66 was in line with the previously reported [6], which demonstrated that the sample of

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UiO-66 with the excellent crystallinity was obtained. After the Pd NPs deposited, the

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peak positions of Pd-U catalysts were in keeping with UiO-66, suggesting that the crystalline structure of UiO-66 was well-maintained. However, the peak intensities of the XRD pattern of Pd-U were reduced, perhaps due to a slight change in the structural regularity of the framework when the Pd NPs were loaded [50,54]. Furthermore, no diffraction peak characteristic of Pd NPs was found in Fig. 1. The 11

absence of Pd peaks could be ascribed to the well-dispersed of Pd NPs or the low Pd loading [55]. The N2 adsorption-desorption isotherms and pore size distribution of UiO-66 and the Pd-U samples were presented in Fig. S1, and the physical structural properties were summarized in Table 1. All the isotherms showed a type Ⅰ pattern, manifesting that all the samples were microporous materials. The BET (Langmuir) specific

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surface area and the total pore volume of UiO-66 were 1335 (1693) m2/g and 0.833

cc/g, respectively [6]. Upon deposition of Pd NPs on the UiO-66 support, the BET

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(Langmuir) specific surface area, total pore volume of the Pd-U-H, Pd-U-NH and Pd-U-EG decreased and the value were 460 (656) m2/g and 0.266 cc/g, 740 (1063)

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m2/g and 0.526 cc/g and 549 (838) m2/g and 0.367 cc/g, respectively. And the pore

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size distribution curves (Fig. S1(B)) of the catalysts determined by Density Functional Theory (DFT) showed that the pore diameter of the catalysts basically unchanged and

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the intensity was weakened after the loading of Pd NPs. The sharply decreased of BET and Langmuir specific surface area, total pore volume and the slight change of

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pore diameter might be due to the Pd NPs occupied or blocked the cavities of the

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UiO-66 support [51, 56].

The scanning electron microscopy (SEM) images of UiO-66, Pd-U-H, Pd-U-NH

and Pd-U-EG were showed in Fig. 2. As could be seen from Fig. 2(a), the UiO-66 with octahedral white crystal and approximately 100 nm in diameter was obtained. The octahedral morphology could also be observed in Pd-U-H, Pd-U-NH and 12

Pd-U-EG catalysts, indicating that the morphology of UiO-66 was not obviously destroyed after loading of Pd NPs. To confirm whether Pd NPs were triumphantly loaded on UiO-66, the transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) was utilized. Fig. 3(a-c) showed the TEM image of Pd-U-H, Pd-U-NH and Pd-U-EG, respectively. As shown in Fig. 3(a), the TEM image

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presented an irregular shape, and no obviously Pd NPs could be observed on the

surface of Pd-U-H. However, Fig. 3(b) and (c) showed that the Pd NPs were highly

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uniform dispersed over the UiO-66. The HR-TEM images of Pd-U-H, Pd-U-NH and

Pd-U-EG were presented in Fig. 3(d-f), respectively. Fig. 3(e) and (f) displayed that

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the lattice spacing of Pd NPs was 0.224 nm, which corresponded to the interplanar

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spacing of Pd (111). Nevertheless, compare with Pd-U-NH and Pd-U-EG, no Pd NPs could be found in Pd-U-H, which might be due to the formation of sub-nanometric

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palladium clusters [57, 58]. As can be seen from the UV-vis diffuse reflectance spectra (Fig. S2), after the deposition of Pd NPs, the photoresponse of the three

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catalysts was shifted to visible light, and the Pd-U-EG displayed a wider absorption

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bands in comparison to Pd-U-NH and Pd-U-H. The shift of photoresponse to visible light might be caused by the introduction of Pd species [59], which indicate that the Pd species were successfully deposited on the Pd-U-H catalyst. To determine the specifics of surface element compositions and chemical state of the Pd-U catalysts, the X-ray photoelectron spectroscopy (XPS) of the catalysts was 13

performed and the results were displayed in Fig. 4. The peak of C, O, N and Zr could be clearly seen from Fig. 4(A), no obvious of the peak of Pd was observed. As we all know, the binding energies of Pd 3d and Zr 3p were very close [50]. It might be that the peak of Zr was too large to cover up the peak of Pd. The Pd 3d and Zr 3p of the three catalysts were depicted in Fig. 4(B), the Zr 3p could be divided into two peaks at 333.6 eV and 327.2 eV corresponding to 3p1/2 and 3p3/2, respectively [50]. The peak

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of Pd 3d in Pd-U-NH and Pd-U-EG were divided into four peaks respectively

corresponding to 3d5/2, 3d3/2 of Pd0 and 3d5/2, 3d3/2 of Pd2+ [50, 52, 60]. Compared

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with Pd-U-NH (Pd0 (3d5/2, 3d3/2) = 336.2, 341.0 eV, Pd2+ (3d5/2, 3d3/2) = 337.5, 344.3 eV), the peaks of Pd in Pd-U-EG were slightly shifted. Interestingly, the peaks of Pd

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3d in Pd-U-H were only divided into three distinct peaks with the binding energies at

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336.2 eV, 338.1 eV and 343.0 eV corresponding to Pd0 3d5/2, Pd2+ 3d5/2 and 3d3/2. The reason for this phenomenon might be that the Pd0 3d3/2 orbital was too weak to detect

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or the signal was too small to be obscured by the peak of Zr 3p. The ratio of Pd0/Pd2+ was list in Table 1 and the Pd-U-EG possesses higher ratio value (0.93) than other

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samples, which were in line with catalytic performance of these catalysts for toluene

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combustion.

The Zr 3d and O 1s XPS were exhibited in Fig. 4(C) and (D), respectively. The

Zr 3d could be divided into two peaks at binding energies of 182.9 eV and 185.3 eV corresponding to 3d3/2 and 3d5/2, respectively [61, 62]. O 1s contained three peaks at 530.3 eV, 531.7eV and 533.3eV respectively attributed to lattice oxygen (Olat), 14

adsorption oxygen (Oads) and surface OH (OOH) [63-66]. The ratio of Olat to Oads was summarized in Table 1. It could be obviously seen that Olat/Oads molar ratio of Pd-U-H, Pd-U-NH and Pd-U-EG increased in turn. The highest Olat/Oads molar ratio (0.164) was observed on Pd-U-EG, which was considered to be favorable to Mars-van-Krevelen mechanism for toluene degradation [67-69]. 3.2 Catalytic performance

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Toluene was chosen to estimate the catalytic activity of the Pd-U catalysts under the conditions of toluene concentration = 1000 ppm, WHSV =30000 mL/g/h. The

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temperature-dependent toluene conversion of UiO-66 and Pd-U catalysts was displayed in Fig. 5. The T10%, T50% and T90% (corresponding to the 10, 50 and 90% of

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toluene conversion) were used to assess the catalytic efficiency, and were exhibited in

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Table S1. It was clearly observed that the pure UiO-66 showed a poorly catalytic activity for toluene combustion, and only presented less than 10% conversion of

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toluene at 400 °C. After Pd NPs encapsulated, the catalytic performance of all the Pd-U catalysts sharply improved. Compared with the pure UiO-66 (T10% > 400 °C),

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the T10% of toluene conversion over Pd-U catalysts reduced more than 200 °C. The

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introduction of Pd was favorable to the improvement of catalytic performance to toluene combustion. The T90% performance of Pd-U-H, Pd-U-NH and Pd-U-EG were respectively corresponding to 238 °C, 204 °C and 198 °C, and maximum difference could reach to 40 °C compared to Pd-U-H. The order of the catalytic efficiency was ranked as follows: Pd-U-EG > Pd-U-NH > Pd-U-H. Table 2 summarized the catalytic 15

activity for toluene combustion of several Pd catalysts. Compared with other Pd catalysts such as Pd@CeO2 /Al2O3 [72], Pd/3DOM Mn2O3 [73], and Pd/OMS-2 [76] etc., the Pd-U catalysts synthesized in this work displayed a better activity for toluene oxidation. Generally speaking, the component, oxygen species, structure, surface area and defect nature were closely related to the catalytic activity of catalyst [15, 77]. It had

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been determined the catalytic efficiency could be significant influenced by preparation methods which had a huge influence to the physicochemical properties of

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the catalysts [60]. Compared with Pd-U-H and Pd-U-NH, the Pd-U-EG possesses

higher active Pd species and lattice oxygen content (Table 1). As we all known,

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abundant Oads species played a significant role in the catalytic oxidation of toluene

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[15-17, 70]. However, some studies have shown that Olat species is also beneficial for the oxidation of toluene [67, 78]. Long time oxygen and reaction gas treatment could

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be beneficial to the formation of lattice oxygen, consequently promoting the catalytic performance [78]. It had been confirmed that the amount of the surface Pd0 species,

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which were serve as the active site, was the one of the main factors to improve the

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catalytic activity [52]. In addition, according to the reports, the toluene could convert to CO2 and H2O by lattice oxygen [79, 80]. Usually, lattice oxygen might be recirculated through direct activation of gaseous oxygen in catalytic reactions. Therefore, the Pd-U-EG presented the highest Olat/Oads, which was favorable to the toluene catalytic oxidation [78, 80]. The optimal catalytic ability of Pd-U-EG could be 16

reasonably attributed to the synergistic effect of Olat species and a high dispersion and high proportion of Pd0/Pd2+. The effect of WHSV, stability and relative humidity were used to further investigate the catalytic performance for toluene degradation. The effect of WHSV on the catalytic activity of the three Pd-U catalysts was depicted in Fig. 6. For the sample of Pd-U-H, when WHSV increased from 30000 to 60000 mL/g/h the temperature of

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toluene completely conversion was only risen about 10 °C. However, as the WHSV

was further increased to 90000 mL/g/h, the Pd-U-H performance unchanged. For

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Pd-U-NH, as the WHSV increased, the catalytic performance decreases. However, the obviously difference from the first two, with the WHSV rising, the catalytic ability of

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Pd-U-EG has no changed, which exhibited a better property to resist the change of

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WHSV. This was different from many catalysts. For example, Chen et al. prepared 0.82Pt@M-Cr2O3 of which the catalytic activity was decreased with the rise of

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WHSV from 20000 to 120000 mL/g/h [81]. The on-stream reaction experiments were carried out to estimate the catalytic stability of the three Pd-U catalysts. As shown in

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Fig. S3, in the premier 1 h, toluene was not completely converted. As the reaction

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time increased, the activity of the catalyst was improved and remained unchanged which indicated that three Pd-U catalysts displayed a good stability to toluene decomposition. The XRD (Fig. S4) of the Pd-U samples after toluene combustion also showed that the samples have a good stability. Given that the actual emission conditions may contain a large amount of 17

moisture, the influence of water vapor onto catalytic performance was studied under the WHSV of 30000 mL/g/h in the intake streams containing 5 vol.%, 10 vol.% and 20 vol.% water. As depicted in Fig. 7, when the 5 vol.%, 10 vol.% and 20 vol.% water vapors were introduced into the reaction system, the conversion of toluene had no decreased, which indicated that no matter water on or off, there was no influence on the catalytic activity of the catalysts. Additionally, in order to further confirm that the

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three catalysts had good water resistance, the temperature-dependent toluene conversion of Pd-U catalysts with the absence and presence 10 vol.% water vapor

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were carried out. As could be seen from Fig. S5, the water vapor had a slight effect on

the conversion of toluene at low temperatures (< 170 °C), and the effect gradually

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decreases and disappears with increasing temperature (about 190 °C), which further

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indicated that the catalyst had good water resistance. In a word, the Pd-U catalysts had good catalytic stabilities and good tolerance to humidity.

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3.3 Reusability test

To detect the recyclability of the Pd-U catalysts, the cyclic catalytic test was

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carried out 6 times. The reusability of Pd-U-H, Pd-U-NH and Pd-U-EG were

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respectively presented in Fig. 6(A), (B) and (C). Noticeable, with the increase of cycle number, the catalytic performance of Pd-U-H gradually improved. When the cyclic experiment carried out for the second time, the temperature of toluene completely conversion decreased about 25 °C. As the number of cycles increased, the catalytic performance would further be improved to 195 °C. The results revealed that the 18

structure of Pd-U-H catalyst was reconstructed by the reaction gas during the toluene oxidation process, which improved the performance of the catalyst. To illustrate this result, N2 adsorption-desorption, TEM and HR-TEM and XPS of Pd-U-H-reused were carried out. The N2 adsorption-desorption isotherms and pore size distribution of Pd-U-H-reused catalysts were displayed in Fig. S6(a), and the physical structural properties were listed in Table S2. Compared with Pd-U-H, the BET (Langmuir)

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specific surface area and total pore volume of Pd-U-H-reused (625 (773) m2/g and 0.324 cc/g) are increased. Fig. S7(a) and (b) showed the TEM and HR-TEM of

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Pd-U-H-reused. The Pd NPs were formed after reaction (Fig. 6(a)), which indicated that the sub-nanometric palladium clusters were aggregated to form the high

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dispersion Pd NPs after the six run catalytic reactions [50]. The HR-TEM image (Fig.

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S7(b)) also displayed that the lattice spacing of Pd NPs was 0.224 nm, which corresponded to the interplanar spacing of Pd (111), also confirmed the formation of

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Pd NPs. To determine the chemical compositions and state of the Pd-U-H-reused, the XPS was performed and the results were listed in Table S2. The peak of O 1s (Fig. S8

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(a)) was divided to three peaks, which in line with Pd-U-H. Differently, compared to

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Pd-U-H, the molar ratio of Olat/Oads of Pd-U-H-reused (0.154) was increased. More importantly, the Pd 3d was divided into four peaks, which was different from the Pd 3d of Pd-U-H. Furthermore, the ratio of Pd0/Pd2+ also improved from 0.77 (Pd-U-H) to 0.89 (Pd-U-H-reused). It had proved that the activity of catalyst could be enhanced by the increase of the Olat which serve as the oxygen vacancies [78] and the surface 19

Pd0 which act as active sites [52] to improve the catalytic performance for toluene combustion. Unlike to Pd-U-H, the reusability of Pd-U-NH and Pd-U-EG showed that the catalytic activity was also improved to some extent. Along with the recycle increasing, the catalytic activity was no longer further improved. To understand the change of Pd-U-NH and the Pd-U-EG before and after the catalytic reaction, the N2

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adsorption-desorption (Fig. S6(b-c)) and XPS (Fig. S8(b-c)) of Pd-U-NH-reused and

Pd-U-EG-reused were also carried out. Same as the Pd-U-H-used, the BET (Langmuir)

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specific surface area and total pore volume of Pd-U-NH-reused (797 (1150) m2/g and

0.686 cc/g) were increased, compared with Pd-U-H. Oppositely, compare with

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Pd-U-EG, the BET (Langmuir) specific surface area and total pore volume of

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Pd-U-EG-used were reduced. This might be due to the partially disordered crystal structure appeared after reaction, which could be got from the XRD results of

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Pd-U-EG-reused (Fig. S4(a)). Consistent with Pd-U-H-reused, the ratio of Olat/Oads and Pd0/Pd2+ (Table S2) of XPS results of Pd-U-NH-reused (0.251 and 1.00) and

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Pd-U-EG-reused (0.473 and 1.06) were also increased, compare with Pd-U-NH and

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Pd-U-EG. In summary, the three Pd-U catalysts had a great reusability. Scheme 1(b-c) show the proposed formation of the three catalysts. For the

Pd-U-H catalyst prepared by pre-impregnation method, when H2 was used to reduce the Pd-U catalyst, the Pd was first reduced to sub-nanometric palladium clusters [57]. And then, the Pd NPs were formed, after reacting with reaction atmosphere. However, 20

when the stronger reductant NaBH4 and EG were used to reduce the Pd-U catalysts, Pd could be reduced in one step to form high dispersion Pd NPs. Different from Pd-U-H, after reaction, the Pd-U-HN had no obviously change (Fig. S4(c)). However, the intensity of XRD (Fig. S(4d)) of Pd-U-EG weakened after reaction, which might be due to that during the reaction, the catalyst interacted with the reaction gas, resulting in crystal defects, weakening the crystallinity. This was in line with the

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decrease in the BET (Langmuir) specific surface area of Pd-U-EG-used.

3.4 Mechanism for toluene combustion and the H2O effect over Pd-U-EG

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3.4.1 Toluene-TPD-MS and toluene-TPSR-MS experiments

In order to understand the toluene adsorption, surface reaction and the effect of

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water on the catalyst, the Pd-U-EG with high catalytic activity was chosen.

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Toluene-TPD experiments (pre-adsorption toluene and O2 or toluene, O2 and H2O) ware performed on Pd-U-EG to explore the final product in the oxidation of toluene.

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Fig. 9 showed the profiles of detected during toluene-TPD. As shown in Fig. 9(A), presence or absence the pre-adsorption of water vapor, the desorption of toluene

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began at about 60 °C，and their peaks appeared at about 165 °C. Interestingly,

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compare with not pre-adsorbed water vapor, the sample which pre-adsorbed water vapor showed a slower desorption process at the beginning of desorption and more toluene was detected. This phenomenon suggested that the introduction of water vapor facilitated the toluene adsorption on Pd-U-EG and slowed desorption of toluene. Additionally, CO2 (Fig. 9(B)), C6H5CHO (Fig. 9(C)) and C4H4O4 (Fig. 9(E)) were 21

also detected during the experiments of toluene-TPD. Since no gaseous oxygen was introduced during the experiments, the formation of CO2 and other byproducts should originate from the reaction between pre-adsorbed oxygen species or the Olat and toluene adsorbed on the surface of catalysts [82]. Fig. 9(D) showed no C6H5COOH was detected in this process. Toluene-TPSR experiments were also carried out under the pre-adsorption

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condition presence or absence of water vapor, and the results were depicted in Fig. 10.

The toluene desorbed in TPSR was presented in Fig. 10(A). It could clearly see that

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the peaks of toluene locate at 144 and 158 °C corresponding to the absence and

presence of water vapor when pre-adsorbing toluene, respectively. This indicated that

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the introduction of water was unfavorable to desorption of toluene, which was in line

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with the results of toluene-TPD (Fig. 9(A)). Additionally, the intensity of the toluene desorption peak demonstrated that the introduction of H2O was favorable to the

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degradation of toluene. Furthermore, the time of the onset of CO2 generation (Fig. 10(B)) and the intensity of the peaks also indicated that the introduction of water

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contributed to the toluene oxidation. Furthermore, the intensity of the peaks of

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C6H5CHO (Fig. 10(C)) and C4H4O4 (Fig. 10(E)) which generated during toluene-TPSR also suggested that the introduction of water could reduce the formation of some by-products. However, compared with the absence of water vapor in the sample, more C6H5COOH (Fig. 10(C)) were generated in the process of toluene-TPSR after water vapor introduced. This might be due to the water 22

penetration increased the content of hydroxyl groups in the sample which promoted the generation of benzoic acid. In short, benzaldehyde, benzoic acid and maleic acid are produced during the process of toluene-TPSR. And the presence of water vapor was conducive to the adsorption and degradation of toluene by the catalyst, which further illustrated that the catalyst had good water resistance. 3.4.2 The in-situ DRIFTS over Pd-U-EG

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In order to further study the intermediates formed during the process of toluene

degradation, the in-situ DRIFTS was performed to examine the possible reaction steps

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on Pd-U-EG and the results were presented in Fig. 11. The information of the

intermediates formed during the process of toluene degradation was listed in Table S3.

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It was clearly seen that the C-H stretching vibrations of the aromatic ring were

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observed at 3027 cm-1, which indicated that the toluene was absorbed on the surface of Pd-U-EG at ambient temperature [78, 81-84]. In addition, it could also be

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confirmed by that the C-H stretching vibrations of methyl group was detected at 2919 cm-1 [78, 81, 83]. Fig. 11(A) showed the in-situ DRIFTS tests of the toluene

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adsorption and reaction at 150 °C. Before reaction, the sample pre-adsorbed toluene

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and oxygen for 1 h, and then the reaction chamber began to warm up and continuously fed toluene and oxygen into the reaction chamber. As the reaction progressed, the peaks located at 3024 cm-1 and 2919 cm-1 which represented toluene gradually weakened and disappeared after 2.3 minutes. During this process, several bands at 1853, 1819, 1745, 1433, 1264, 1240 and 23

915 were instantly identified. The band at 1745 and 1240 cm-1 were ascribed to the C=O stretching vibration of aldehydic [78], indicting the formation of benzaldehyde. Additionally, the bands appearing at 1819, 1433 and 1264 cm-1 were characteristic of the stretching vibration of C-O, which were the typical carboxylate group [78, 83], demonstrating the existence of benzoic acid in the intermediates. Band approximately at 1853 cm−1 was ascribed to five-membered cyclic anhydride species [83].

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Combining with the 915 cm−1 band, maleate acid species could be identified [83], which were important intermediates from the aromatic ring breakage process.

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Therefore, according to the in-situ DRIFTS, the reaction mechanism of toluene

combustion might be that toluene rapid transform to benzaldehyde followed by

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benzoate and maleic acid, eventually decayed to CO2 and H2O, which was in line with

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the toluene-TPSR results. And the degradation process was depicted in Scheme 2. In addition, in order to understand the effect of H2O on the toluene degradation,

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the in-situ DRIFTS tests of the toluene adsorption and reaction at 150 °C under the condition with H2O, the results were displayed in Fig. 11(B). Different from the

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impermeability of H2O at 150 °C, the peak which represented toluene did not

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disappear until 3.8 minutes, which suggested that the introduction of water slowed the oxidation process of toluene. However, when the in-situ DRIFTS was tested at 190 °C (Fig. 11(C-D)), there was no significant difference in the degradation of toluene during the reaction presenting the water or not, which indicated that the water vapor had no effect on the catalyst at 190 °C. In short, the water vapor had a little effect on 24

the conversion of toluene at low temperatures, when the increase of the temperature, the influence of water vapor was disappeared. This was consistent with the water resistance of the Pd-U-EG catalyst presented in Fig. S5. Therefore, based on the literature [85, 86] and combining the results of toluene-TPD (Fig. 9(A)) and toluene-TPSR (Fig. 9(B)), we believe that the introduction of water was conducive to the adsorption of toluene and slowed the desorption of toluene, which inhibited the

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degradation of toluene at low temperature. However, when the temperature rose, the

introduction of water could cause the occurrence of hydroperoxyl-like, which was

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conducive to the activation of oxygen [85, 86], facilitating the formation of lattice

oxygen, consequently promoting the catalytic performance [78]. A further study on

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the role of water needs to be carried out in the future.

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4. Conclusion

In summary, we investigated the effects of different preparation processes on the

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formation of Pd nanoparticles and studied the reaction mechanism of toluene on Pd-U-EG and the effect of water introduction on the catalyst by toluene-TPD,

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toluene-TPSR and the in-situ DRIFTS. The highly active Pd-U catalysts were

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successfully synthesized via pre-impregnation method, NaBH4 reduction and EG reduction and compared with Pd-U-H and Pd-U-NH, the Pd-U-EG can completely convert toluene at 200 °C, presenting an excellent catalytic performance for toluene oxidation. It is mainly ascribed to the highly dispersion Pd NPs with higher surface content and more lattice oxygen, which also gave rise to the good thermal stability 25

and excellent reusability. In addition, during the six-run cyclic reaction, Pd-U-H formed Pd NPs, increased the surface Pd0 content and the lattice oxygen content, which greatly enhanced the activity of catalytic oxidation of toluene. Simultaneously, the tolerance to water test, the toluene-TPD and toluene-TPSR in different conditions indicate that the catalyst Pd-U-EG has a great endurability to high moisture (about 10-20 vol.%) and the introduction of water could improve the absorption to toluene.

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More importantly, the study of toluene-TPD, toluene-TPSR under the condition of presence and absence of H2O and the in-situ DRIFTS at 150 and 190 °C with or

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without H2O over Pd-U-EG indicated that at low temperature, the introduction of H2O

was conducive to the adsorption of toluene, slowed the desorption of toluene and

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inhibited the degradation of toluene. However, at high temperature, the H2O presence

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was favorable to toluene degradation. In addition, the reaction mechanism of toluene combustion also revealed. Toluene rapid transform to benzaldehyde followed by

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benzoate and maleic acid, eventually decayed to CO2 and H2O.

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Acknowledgements

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This work was sponsored financially by the Natural Science Foundation of Shanghai (19ZR1434900) and National Natural Science Foundation of China (No.21906104). Thanks to Anhui kemi machinery technology Co., Ltd for providing Teflon-lined stainless steel autoclave.

26

Credit author statement

Fukun Bi: Writing- Original draft preparation Xiaodong Zhang: Writing- Reviewing and Editing, Conceptualization, Funding acquisition Jinfeng Chen: Data curation

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Yang Yang: Visualization, Investigation

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Yuxin Wang: Software, Validation, Funding acquisition

Declaration of interests

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Fukun Bi, Xiaodong Zhang, Jinfeng Chen, Yang Yang, Yuxin Wang

27

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ammonia to nitrogen, Sep. Purif. Technol. 209 (2019) 1016-1026.

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ammonia, ACS Appl. Mater. Interfaces 11 (2019) 46875-46885. [65] C.H. Zhang, K. Zeng, C. Wang, X.H. Liu, G.L. Wu, Z. Wang, D. Wang, LaMnO3

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perovskites via a facile nickel substitution strategy for boosting propane combustion

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[70] Z.X. Wu, J.G. Deng, S.H. Xie, H.G. Yang, X.T. Zhao, K.F. Zhang, H.X. Lin, H.X.

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Dai, G.S. Guo, Mesoporous Cr2O3-supported Au-Pd nanoparticles: High-performance

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Jo

ur

na

lP

re

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ro of

and supported gold nanoclusters, Phys. Rev. Lett. 95 (2005) 9-13.

41

Table 1. Physicochemical parameters of UiO-66 and different Pd-U catalysts.

Samples

XPS

SLangmuir

V(cc/g)

(m2/g)a

(m2/g)b

UiO-66

1335

1693

0.833

Pd-U-H

460

656

Pd-U-NH

740

Pd-U-EG

549

c

d

D(nm)

Pd0/Pd2+

Olat/Oads

0.8-1.5

/

/

0.266

0.9-1.4

0.77

0.133

1063

0.526

0.9-1.5

0.86

0.158

838

0.367

0.9-1.8

ro of

SBET

0.93

0.164

BET specific surface;

b

Langmuir specific surface;

c

Total pore volume measured at P/P0 =0.99;

d

The pore diameter calculated from the desorption branch of the isotherm using the DFT method;

Jo

ur

na

lP

re

-p

a

42

Table 2. Comparison of the activities of different Pd-based catalysts for toluene combustion. Pd Catalysts

loading

Preparation/Reduction

Pretreatment

Reaction

Toluene

method

condition

condition

conversion

32000ml/g/h

100%,

Pd/ZSM-25

1

Impregnation

H2,480°C,2h

Pd/meso-Cr2O3

1

NaBH4-reduction

Air,400°C,4h

0.99

NaBH4-reduction

Air,450°C,4h

1

NaBH4-reduction

Air,500°C,6h

1.9

NaBH4-reduction

Air,300°C,2h

ro of

(wt%)

Ref.

Pd-SBA-15

1

Impregnation

H2,480°C,1h

105000ml/g/h

Pd–CoAlO

3.35

/

Pd/OMS-2

1

Mn2O3

Pd-U-H

precipitation

Air,400°C,2h

Impregnation

H2,200°C,2h

1

NaBH4-reduction

Ar,100°C,1h

1

Ethylene

glycol-reduction

Ar,100°C,1h

Jo

Pd-U-EG

Air,600°C,4h

1

ur

Pd-U-NH

Deposition

1000ppm,

90%,

20000ml/g/h

210°C

1000ppm,

90%,

40000ml/g/h

218°C

30000ml/g/h 1000ppm,

-p

Pd/3DOM

re

Pd@CeO2/Al2O3

lP

Co3O4

na

Pd/3DOM

43

240°C

40000ml/g/h

100%, 225°C 90%,

240°C 90%, 242°C

39

70

71

72

73

74

2000ppm,

90%,

60000ml/g/h

226°C

2000ppm,

100%,

240000ml/g/h

320°C

1000ppm,

90%,

This

30000ml/g/h

238°C

work

1000ppm,

90%,

This

30000ml/g/h

204°C

work

1000ppm,

90%,

This

30000ml/g/h

198°C

work

75

76

Figure captions Fig. 1 XRD patterns of (a) UiO-66, (b) Pd-U-H, (c) Pd-U-NH and (d) Pd-U-EG. Fig. 2 SEM images of the different catalysts: (a) UiO-66, (b) Pd-U-H, (c) Pd-U-NH and (d) Pd-U-EG. Fig. 3 TEM and HR-TEM images of Pd-U-H (a, d), Pd-U-NH (b, e) and Pd-U-EG (c, f).

ro of

Fig. 4 XPS spectra of different Pd-loaded catalyst (a) Pd-U-H, (b) Pd-U-NH and (c) Pd-U-EG in (A) all spectra, (B) Pd 3d Zr 3p, (C) Zr 3d and (D) O 1s.

-p

Fig. 5 Toluene conversion over different catalysts.

Fig. 6 The effect of water vapor on catalytic activity of (A) Pd-U-H, (B) Pd-U-NH

re

and (C) Pd-U-EG.

lP

Fig. 7 The effect of different WHSV on catalytic activity of (A) Pd-U-H, (B) Pd-U-NH and (C) Pd-U-EG.

na

Fig. 8 Reusability of (A) Pd-U-H, (B) Pd-U-NH and (C) Pd-U-EG. Fig. 9 Toluene-TPD-MS profiles of (A) C7H8, (B) CO2, (C) C6H5CHO, (D)

ur

C6H5COOH and (E) C4H4O4 during toluene-TPD process of Pd-U-EG (co-adsorption

Jo

of Toluene + O2, Toluene + O2 + H2O). Fig. 10 Toluene-TPSR-MS profiles of (A) C7H8, (B) CO2, (C) C6H5CHO, (D) C6H5COOH

and

(E)

C4H4O4

during

toluene-TPSR

process

of Pd-U-EG

(co-adsorption of Toluene+O2, Toluene+O2+H2O). Fig. 11 In-situ DRIFTS spectra of Pd-U-EG under (A) 20% O2/Ar + C7H8 and (B) 20% 44

O2/Ar + C7H8 with H2O at 150 °C, (C) 20% O2/Ar + C7H8 and (D) 20% O2/Ar + C7H8 with H2O at 190 °C. Scheme 1 Schematic of synthesis processes of (a) UiO-66, (b) Pd-U-H and (c) Pd-U-NH and Pd-U-EG.

Jo

ur

na

lP

re

-p

ro of

Scheme 2 Mechanism for toluene combustion over Pd-U-EG.

45

Intensity (a.u.)

Fig. 1

(d)

(c)

ro of

(b)

(a)

10

20

30

50

40

60

Jo

ur

na

lP

re

-p

2 Theta (°)

70

46

80

ro of

-p

re

lP

na

ur

Jo Fig. 2

47

ro of

-p

re

lP

na

ur

Jo Fig. 3

48

1200

1000

800

600

Zr 3p C 1s

Zr 3d

Zr 3p C 1s

Zr 3d

(B) Pd 3d Zr 3p

400

Intensity (a.u.)

(a)

200

3p1/2

3p1/2

(c)

3d3/2

0

Oads

OOH

(a) Intensity (a.u.)

(b)

Oads

OOH

(b)

3d5/2

(c)

188

Olat

Oads

OOH

(c) 190

Olat

-p

3d5/2

186

184

182

180

536

Jo

ur

na

lP

B.E. (ev)

re

Intensity (a.u.)

(a)

3d3/2

325

ro of

(D) O 1s 3d5/2

3d3/2

330

335

B.E. (eV)

(C) Zr 3d 3d3/2

3p3/2

3d5/2

340

345

350

B.E. (ev)

3p3/2

3d5/2

3d3/2

3p1/2

3p3/2

3d5/2

3d3/2

(b)

Zr 3d

Zr 3p C 1s

(c)

Zr 3s

O 1s

(b)

O 1s

Intensity (a.u.)

(a)

Zr 3s

O 1s

(A)

Zr 3s

Fig. 4

49

535

534

533

532

B.E. (eV)

Olat

531

530

529

528

Fig. 5 UiO-66 Pd-U-H Pd-U-NH Pd-U-EG

80

60

40

20

0 100

150

200

300

250

350

400

Jo

ur

na

lP

re

-p

Temperature (°C)

ro of

Toluene conversion (%)

100

50

450

Fig. 6

Toluene conversion (%)

100 (A)

80

60

30000 mL/g/h 60000 mL/g/h 90000 mL/g/h

40

20

0 160

140

180

200

220

Temperature (°C)

60

30000 mL/g/h 60000 mL/g/h 90000 mL/g/h

re

40

260

-p

80

20

lP

Toluene conversion (%)

100 (B)

240

ro of

120

100

80

0

100

80

120

140

160

200

180

220

240

na

Temperature (°C)

80

Jo

ur

Toluene conversion (%)

100 (C)

60

40

30000 mL/g/h 60000 mL/g/h 90000 mL/g/h

20

0 100

120

160

140

Temperature (°C)

51

180

200

220

Fig. 7 (A)

Toluene conversion (%)

100

80

60

H2O-on H2O-on H2O-on H2O-off 5 vol.% 10 vol.% 20 vol.%

H2O-free 40

20 Reaction temperature=240°C WSHV=30000 mL/g/h 0 3

6

9

12

15

ro of

0

Time (h)

(B)

-p

80

60

H2O-on H2O-on H2O-on H2O-off 5 vol.% 10 vol.% 20 vol.%

H2O-free 40

re

Toluene conversion (%)

100

20

lP

Reaction temperature=210°C WSHV=30000 mL/g/h

0

6

3

0

9

12

15

na

Time (h) (C)

80

Jo

ur

Toluene conversion (%)

100

60

H2O-on H2O-on H2O-on H2O-off 5 vol.% 10 vol.% 20 vol.%

H2O-free

40

20 Reaction temperature=200°C WSHV=30000 mL/g/h 0 0

3

9

6

Time (h)

52

12

15

Fig. 8

Toluene conversion (%)

100

(A)

80

60

40

1run 2run 3run 4run 5run 6run

20

100

120

140

160

180

200

220

Temperature (°C)

re

60

40

20

0

120

na

100

100

240

260

-p

80

lP

Toluene conversion (%)

100 (B)

ro of

0

140

160

180

1run 2run 3run 4run 5run 6run 200

220

Temperature (°C)

(C)

Toluene conversion (%)

Jo

ur

80

60

40

1run 2run 3run 4run 5run 6run

20

0 100

120

140

160

Temperature (°C)

53

180

200

220

Fig. 9 4

(A)

(B)

CO2 MS signal (a.u.)

C7H8 MS signal (a.u.)

6

2

C7H8+O2 C7H8+O2+H2O

4

C7H8+O2 C7H8+O2+H2O

2

0 0

100

200

500

400

300

0.02

200

300

400

Temperature (°C)

(E)

0.08

0.00

500

100

na

0.06

0.04

0.02

C7H8+O2 C7H8+O2+H2O

ur

0.00

C7H8+O2 C7H8+O2+H2O

lP

0.10

0.01

re

C7H8+O2 C7H8+O2+H2O

100

200

300

400

(D)

-p

0.04

0.00

C4H4O4 MS signal (a.u.)

C6H5COOH MS signal (a.u.)

C6H5CHO MS signal (a.u.)

0.08

ro of

Temperature (°C)

(C)

100

500

400

300

200

100

Temperature (°C)

500

Jo

Temperature (°C)

54

200

300

Temperature (°C)

400

500

Fig. 10 (A)

(B)

144 °C

50

CO2 MS signal (a.u.)

C7H8 MS signal (a.u.)

158 °C 2

1

C7H8+O2 C7H8+O2+H2O

40 30 20

C7H8+O2 C7H8+O2+H2O

10 0

0 200

100

400

300

100

500

(D)

C7H8+O2 C7H8+O2+H2O

0 400

(E)

500

100

na

0.4 C7H8+O2 C7H8+O2+H2O 0.2

100

Jo

0.0

ur

C4H4O4 MS signal (a.u.)

0.03

lP

Temperature (°C)

0.06

re

C7H8+O2 C7H8+O2+H2O

300

200

300

500

400

-p

C6H5COOH MS signal (a.u.)

C6H5CHO MS signal (a.u.)

4

200

400

300

ro of

(C) 8

100

200

Temperature (°C)

Temperature (°C)

500

Temperature (°C)

55

200

300

Temperature (°C)

400

500

8.3min 6.8min 5.3min 3.8min

1436 1263 1243

1847 1816 1742

H2O

3027 2921

0.1

915

3024 2919

(A)

(B)

9.8min

Absorbance (a.u.)

Absorbance (a.u.)

No H2O

1853 1819 1745

0.1

1433 1264 1240

Fig. 11

8.3min 6.8min 5.3min 3.8min 2.3min

2.3min 0.8min 150°C C7H8+H2O+O2

0.8min 150°C C7H8+O2 30°C C7H8+O2

30°C C7H8+H2O+O2

1500

3500

4000

1000

0.1

919

1436 1264 1240

1846 1817 1750

H2O Absorbance (a.u.)

6.8min 5.3min 3.8min

8.3min 6.8min 5.3min 3.8min 2.3min

2.3min 0.8min 190°C C7H8+O2

3000

2000

1500

1000

4000

Jo

ur

na

lP

Wavenumber (cm-1)

re

0.8min 190°C C7H8+O2+H2O

30°C C7H8+O2 3500

(D)

-p

3027 2919

Absorbance (a.u.)

(C)

8.3min

4000

1000

Wavenumber (cm )

Wavenumber (cm )

No H2O

1500

2000

-1

-1

0.1

3000

1434 12641240

2000

1847 1818 1750

3000

ro of

3500

3027 2919

4000

56

3500

30°C C7H8+O2

3000

2000

1500

Wavenumber (cm-1)

1000

ro of

-p

re

lP

na

ur

Jo Scheme 1

57

ro of

-p

re

lP

na

ur

Jo Scheme 2

58