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cordierite bimetallic monolithic catalysts
Accepted Manuscript Title: Hydrogenation of 2-ethylanthraquinone on Pd-La/SiO2 /cordierite and Pd-Zn/SiO2 /cordierite bimetallic monolithic catalysts Authors: Yanyan Guo, Chengna Dai, Zhigang Lei PII: DOI: Reference:
S0255-2701(18)30342-8 https://doi.org/10.1016/j.cep.2018.11.006 CEP 7421
To appear in:
Chemical Engineering and Processing
Received date: Revised date: Accepted date:
21 March 2018 30 October 2018 12 November 2018
Please cite this article as: Guo Y, Dai C, Lei Z, Hydrogenation of 2ethylanthraquinone on Pd-La/SiO2 /cordierite and Pd-Zn/SiO2 /cordierite bimetallic monolithic catalysts, Chemical Engineering and Processing - Process Intensification (2018), https://doi.org/10.1016/j.cep.2018.11.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hydrogenation of 2-ethylanthraquinone on Pd-La/SiO2/cordierite and Pd-Zn/SiO2/cordierite bimetallic monolithic catalysts Yanyan Guoa, Chengna Daib, Zhigang Leib,* a
*Corresponding
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Department of Medicinal Chemistry, School of Pharmacy, Anhui University of Chinese Medicine, Hefei 230031, China bState Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Energy Environmental Catalysis, Beijing University of Chemical Technology, Box 266, Beijing 100029, China
author. Tel: +86 10 64433695.
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E-mail address: [email protected] (Z. Lei).
Graphical abstract
and selectivity (95%) for the hydrogenation of 2-ethylanthraquinone with
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g·L-1)
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0.8% Pd-0.4% La/SiO2/cordierite monolithic catalyst exhibits the highest H2O2 yield (7.7
M
A
anthraquinone method.
eAQH2
C
O
H
A
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ED
eAQ
H2
1
Pd
La
Highlights
A series of Pd-La(Zn)/SiO2/cordierite bimetallic monolithic catalysts were prepared;
0.8%Pd-0.4%La/SiO2/COR catalyst is superior to 1.2%Pd/SiO2/COR due to
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synergistic effect; The mechanism of eAQ-PdM (M = La and Zn) surface interactions was explored by
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DFT calculations.
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Abstract A series of Pd-La/SiO2/cordierite (cordierite = COR) and Pd-Zn/SiO2/COR
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bimetallic monolithic catalysts were prepared, characterized, and tested for the hydrogenation
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of 2-ethylanthraquinone (eAQ) as the key step of anthraquinone method for production of
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H2O2. It was found that the metal particle size is remarkably dependent on the Pd/M (M = La, and Zn) mass ratio of bimetallic monolithic catalysts. By virtue of its small metal particle size
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and the strong interaction, the 0.8% Pd-0.4% La/SiO2/COR monolithic catalyst presents the
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highest H2O2 yield (7.7 g·L-1) and selectivity toward active quinones (95%), which is attributed to the addition of La leading to the synergistic effect between Pd and La. Moreover, the density functional theory (DFT) calculations for the adsorption of eAQ over the surface of
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Pd-La/SiO2/COR and Pd-Zn/SiO2/COR bimetallic monolithic catalysts have been conducted to further understand the mechanism of molecule–surface interactions between eAQ and the PdM(1 1 1) or Pd(1 1 1) surfaces. Detailed studies showed that the high catalytic activity of Pd-La/SiO2/COR bimetallic monolithic catalyst is attributed to the stronger adsorption
2
between Pd3La1 (1 1 1) and the carbonyl group of eAQ. Keywords: Bimetallic monolithic catalysts; eAQ hydrogenation; DFT calculation; Synergistic effect
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1. Introduction Hydrogen peroxide (H2O2) is an important green chemical raw material and
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environmental friendly oxidizing agent in many industry areas [1-4]. Up to now, more than
98% of H2O2 is manufactured by the anthraquinone process. In this cyclic process, the (eAQ)
is
first
hydrogenated
to
the
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2-ethyl-9,10-anthraquinone
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2-ethyl-9,10-anthrahydroquinone (eAQH2), and then the eAQH2 is oxidized to produce eAQ
products,
but
2-ethyl-anthrone
(eAN),
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desired
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and H2O2 (see Scheme 1). In the catalytic hydrogenation of eAQ, eAQH2 and H4eAQH2 are 2-ethyloxoanthrone
(OXO),
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2-ethyl-tetrahydro-anthrone (H4eAN), and octahydroanthrahydroquinone (H8eAQH2) are undesired products, which are attributed to the deep hydrogenation of eAQH2 [5]. Therefore,
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eAQ and 2-ethyl-5,6,7,8-tetrahydro-9,10-anthraquinone (H4eAQ) are active quinones.
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Reactions to produce H2O2
O Et
+ O
H2O2
eAQ H2
O2 OH
OH Et
OH
H2
Et
OH
eAQH2
H4eAQH2 H2 OH
O
Et
Et
H
OH
OH
OXO H2
H8eAQH2 3
O
These four products can not produce H2O2
Et
eAN
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Scheme 1. The reaction route of the anthraquinone hydrogenation/oxidation process to produce H2O2 [6].
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Supported Pd catalysts are the most commonly used catalysts in the hydrogenation of eAQ, which is attributed to the supported Pd catalysts presenting better catalytic activities
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than other pellet catalysts in the hydrogenation of eAQ. However, mass transfer is the
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controlling factor in the catalytic hydrogenation of eAQ [7-11]. In principle, the internal and
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external diffusion resistances decrease when using monolithic catalysts, and thus the overall
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reaction rate can be improved. Thus, the supported Pd monolithic catalysts were investigated
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in the hydrogenation of eAQ in previous publications. Moreover, the studies are mainly focused on modifying the supports of monolithic catalysts to improve the catalytic activities
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[12-14]. For example, the Pd/Al2O3/COR has higher selectivity and H2O2 yield than the
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Pd/Al2O3 pellet catalyst in the catalytic hydrogenation of eAQ, due to the monolithic catalyst having short diffusion distances [12]. The Pd/SiO2/COR monolithic catalyst has higher catalytic stability than the Pd/Al2O3/COR monolithic catalyst in the catalytic hydrogenation
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of eAQ, due to the Pd/SiO2/COR monolithic catalyst having more regular structure and weaker acidity [13]. In our previous work, the Pd/SiO2/COR monolithic catalyst have higher selectivity and H2O2 yield than Pd/Al2O3 pellet catalyst, due to the high mass transfer coefficient [6,14].
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In addition to modifying the support, the modification of the active sites for the catalysts (i.e., morphology and composition) can improve the catalytic performance. For example, the catalytic activity and selectivity of Pd-Fe/SiO2 bimetallic catalyst were higher than those of Pd/SiO2 catalysts in the Liquid-phase hydrogenation of phenylacetylene to styrene [15].
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Pd-Cu bimetallic catalyst presents higher conversion of furfural and higher selectivity (> 98%) than Pd/SiO2 catalyst [16]. The higher catalytic performance of such hybrid materials is due
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to the synergistic inter-metallic interactions between Pd and the second metal [17-19]. Recently, the combination of Pd with La or Zn has attracted particular attention. For example,
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the catalytic activity and selectivity of Pd-Zn/TiO2 and Pd-Zn/CeO2 catalysts are higher than
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those of Pd/TiO2 in semi-hydrogenation of acetylene due to the synergistic interactions
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between Pd and Zn of PdZn alloy [20,21]. Pd-La/spinel was tested for the preparation of
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2,6-diisopropylaniline. It showed that the activity and selectivity of Pd-La/spinel increase
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when compared to Pd catalyst through neutralizing the strong acid sites on the surface of catalyst and decreasing the formation of carbon [22]. Therefore, the synergistic inter-metallic
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interactions between Pd and La or Zn over Pd-based bimetallic monolithic catalysts offer the
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promise for simultaneously increasing both selectivity and H2O2 yields in the hydrogenation of eAQ.
Therefore, the present work focuses on improving the H2O2 yields and selectivity in the
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hydrogenation of eAQ by adding the La, or Zn into Pd/SiO2/COR monolithic catalyst. In this work, a series of Pd, La, and Zn monometallic, and Pd-M (M = La or Zn) monolithic catalysts were prepared with the co-impregnation method for the catalytic hydrogenation of eAQ. Then, transmission electron microscopy (TEM), scanning electron microscopy (SEM),
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X-ray photoelectron spectroscopy (XPS), H2 temperature programmed reduction (H2-TPR), and H2–O2 titration were conducted to investigate the structure and composition of monolithic catalysts, thus revealing the synergistic effect between Pd and the second metal (M= La, Zn). Finally, a deep mechanistic understanding for the hydrogenation of eAQ on
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different monometallic and bimetallic monolithic catalysts was performed by DFT calculations.
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2. Experimental section 2.1. Materials
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Cordierite was purchased from Nanning Yilaite Environmental Protection Technology
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Co. PdCl2 (99.5 wt%) was purchased from Tianjin Guangfu Fine Chemical Research Institute.
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C4H6O4Zn·2H2O (99 wt%) and LaCl3·7H2O (99.9 wt%) were purchased from Shanghai
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Aladdin Chemistry Co. SiO2 sol (30 wt%) was purchased from Sigma Aldrich (Shanghai)
2.2. Catalyst preparation
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trading Co., Ltd.
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2.2.1. Washcoating of monoliths
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Because the surface area of bare monoliths is low, they can’t provide sufficient active sites for active components. Thus, the monoliths were coated a layer of SiO2 sol by impregnation method to increase the surface area of monolith supports. The cylindrical
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cordierite ( 132× 70 mm and 400 channels per square inch (cpsi)) were cut into pieces of monolithic supports ( 20 × 10 mm). The supports were heated in 15 wt.% nitric acid for 4 h at 80 °C, and then washed by the distilled water to neutrality. Afterward, the supports pretreated with nitric acid were dried in an oven at 100 °C for 2 h, and then calcined in a
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muffle furnace at 550 °C for 4 h. After acid pretreatment, the monolith supports were dipped into 30 wt% SiO2 sol solution for 5 min, and then the coated monolith supports were dried in an oven for 15 min at 100°C. Before the desired loading of monolith supports were obtained, the steps were repeated several times. Finally, the monolith supports with the desired loading
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were calcined in a muffle furnace at 550 °C for 4 h. 2.2.2. Metal deposition
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A series of Pd, La, and Zn monometallic, and Pd-M bimetallic monolithic catalysts were
prepared by co-impregnation method. The coated monolith supports were impregnated into
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the PdCl2, LaCl3, and Zn(CH3COOH)2 aqueous solutions at 100 °C for 24 h to obtain the
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precursors of Pd/SiO2/COR, La/SiO2/COR, and Zn/SiO2/COR. The PdCl2 and LaCl3 aqueous
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solutions were mixed, and the mixed solution was stirred for 30 min. Meanwhile, the PdCl2
M
and Zn(CH3COOH)2 aqueous solutions were mixed, and the mixed solution was stirred for 30
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min. Then, the coated monolith supports were impregnated into the two mixed solutions at 100 °C for 24 h to obtain the precursors of Pd-M bimetallic monolithic catalysts. The
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precursors Pd/SiO2/COR, La/SiO2/COR, Zn/SiO2/COR monometallic and Pd-M/SiO2/COR
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bimetallic monolithic catalysts were calcined in an muffle furnace at 550 °C for 4 h to obtain the oxide species of Pd/SiO2/COR, La/SiO2/COR, Zn/SiO2/COR monometallic and Pd-M/SiO2/COR bimetallic monolithic catalysts. Prior to the catalytic experiment, these
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oxide species of bimetallic monolithic catalysts were reduced by H2 gas to obtain the Pd/SiO2/COR, La/SiO2/COR, Zn/SiO2/COR monometallic and Pd-M/SiO2/COR bimetallic monolithic catalysts. Herein, the loading amount of SiO2 washcoat was calculated based on all of the
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monolithic catalyst weight including COR and SiO2, while the loading amount of the metal (Pd, La or Zn) analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) was calculated based on the weight of SiO2 washcoat. 2.3. Catalyst characterization
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The washcoat of monolithic catalysts was observed by scanning electron microscopy (SEM) (JSM-6701F, JEOL, Japan), which was operated at an accelerating voltage of 5.0 kV.
ion sputtering instrument to make the samples conductive.
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Prior to observation, the surface of the samples was sprayed a thin layer of gold film using an
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Transmission electron microscopy (TEM) (JEM-2000 EX, JEOL, Japan), conducted at
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an accelerating voltage of 120 kV, was used to measure the particle size and morphology of
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monolithic catalysts. Prior to observation, the samples of monolithic catalysts were ground
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into powder and then dispersed uniformly in the ethanol solution by ultrasonication. Drops of
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the suspension were dipped onto a transparent carbon foil supported by copper grid and dried in air.
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Hydrogen temperature programmed reduction (H2-TPR) curves of monolithic catalysts
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were measured to investigate the reducibilities of monolithic catalysts using a Thermo Electron TPD/R/O 1100 series instrument equipped with a thermal conductivity detector (TCD).
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The composition and chemical state of active species on the surface of monolithic
catalysts were measured by X-ray photoelectron spectroscopy (XPS) spectra by an photoelectron spectrometer (ESCALAB 250, thermoFisher Scientific, USA) using C as the internal standard (C1s = 284.6 eV).
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ICP-AES (ICP-7500, Shimadzu Corporation, Japan) was used to measure the actual Pd and the second metal contents of monolithic catalysts. To investigate the Pd dispersion degree and Pd surface area of Pd-M/SiO2/COR bimetallic monolithic catalysts after the addition of the second metal (La or Zn), the H2–O2
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titration was performed on a Thermo Electron TPD/R/O 1100 series instrument, which was equipped with a thermal conductivity detector (TCD). The details on operating procedure can
were calculated by [23]:
S Pd ( m g
1
)
100
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3 2 2 .4 W P
2 VH N 10 3 2 2 .4 W P
(1)
-3
100
(2)
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2
-3
N
D P d (% )
2 VH M 10
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be found in our previous work [6]. The Pd dispersion (DPd) and Pd metal surface area (SPd)
M
where VH is the volume of H2 used for the titration of O2 (mL); M is the relative molecular
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weight of Pd (g·mol-1); W is the catalyst weight (g); P is the mass fraction of Pd; N (= 6.023 × 1023) is Avogadro’s constant; and σ is the cross-sectional area of a Pd atom (8.97 × 10−20 m2).
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2.4. Catalytic activity test
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The catalytic performance of Pd/SiO2/COR, La/SiO2/COR, Zn/SiO2/COR, and Pd-M/SiO2/COR monolithic catalysts were tested in the catalytic hydrogenation of eAQ, performed in a stainless steel fixed-bed reactor (20 mm in inner diameter and 450 mm in
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height). Prior to reaction, 3.6 g eAQ was dissolved in 60 mL working solution, which is composed of 15 mL trioctyl phosphate and 45 mL industrial grade C9 aromatics, to obtain the new working solution. Ten pieces of monolithic catalysts ( 20 × 10 mm) were fixed in the reaction zone of fixed-bed reactor. The precursors of monolithic catalysts were first reduced
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at a certain temperature for 3 h to obtain the monolithic catalysts. Then, the catalytic hydrogenation of eAQ was performed at 80 °C, while the H2 gas (99.999 wt%) of 100 mL·min-1 and the working solution of 10 mL·min-1 were fed into the fixed-bed reactor. The details on operating parameters can be found in our previous work [6].
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After hydrogenation, 5 mL of the hydrogenated working solution was placed into the oxidation device, and oxidized at 25 °C and 101 kPa for 30 min with the air produced from
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air generator (GC-ready SPB-5000 Automatic Air Source, China). Then, the working solution
was extracted by distilled water to obtain H2O2 solution. Prior to the titration, the 15 mL
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sulfuric acid solution (25 wt%) were placed into the H2O2 solution. Finally, the amount of
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H2O2 was titrated using the potassium permanganate solution.
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The H2O2 yield (B), catalyst selectivity (S), and turnover frequency (TOF) were
V
5C
K M nO 4
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B
M
calculated by
S
n eA Q n H
4
eA Q
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H 2O 2
(3)
2V 1 0 0 %
n0 (eA Q )
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M
K M nO 4
nH
2O 2
1 0 0 %
(4)
n0 (eA Q )
TOF
n0C
(5)
n cat t
n cat
WP M
D Pd
(6)
Pd
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where CKMnO4 is the concentration of KMnO4 solution (mol·L-1); and VKMnO4 and V represent the volumes of KMnO4 solution and H2O2 solution (mL), respectively; S is the selectivity toward active quinones (eAQ and H4eAQ); n0 and n are the molar concentrations of components in the initial working solution and oxidized solution (mol·L-1), respectively;
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nH
is the molar concentration of component of H2O2 in the oxidized solution (mol·L-1). C is
2O 2
the conversion of eAQ at the reaction time t; ncat is the mole number of exposed Pd atom; MPd is the molar mass of Pd; W is the catalyst weight (g); P is the mass fraction of Pd; and DPd is the Pd dispersion.
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2.5. Computational method Density functional theory (DFT) calculations, based on the first-principles, were
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performed to investigate the optimized surface structures of catalysts and the adsorption energy for adsorbate on the optimized surface of catalysts [24,25]. In this work,
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spin-polarized periodic DFT calculations were conducted using the Vienna Ab-Initio
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Simulation Package (VASP). The valence electrons with a cutoff energy of 400 eV were
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described by plane wave basis sets, and the core electrons were replaced by the projector
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augmented wave (PAW) pseudo-potentials [26,27]. The exchange–correlation functional was
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described using the generalized gradient approximation (GGA), which was proposed by Perdew, Burke, and Ernzerhof (PBE) [28]. The sampling of the Brillouin-zone for p(3 × 2)
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lateral supercell was restricted to a 2 × 2 × 1 Monkhorst-Pack grid. The criterion of
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convergence is less than 0.05 eV·Å−1 and 4 × 10−6 eV for the force and energy differences, respectively.
In the calculation, the Pd3M1 (M = La or Zn), along with pure Pd, was applied for
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modeling the Pd3M1 alloys. The optimized lattice constants are a = b = c = 3.94 Å for Pd, a = b = c = 4.22 Å for Pd3La1, and a = b = c = 3.92 Å for Pd3Zn1, consistent with the values reported in literature [29,30]. The closed packed (3 2) Pd(111), Pd3La1(111) and Pd3Zn1(111) surfaces were modeled using a 3-layer Pd slab, in which the atoms in the bottom two layers
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are fixed, while the atoms in the top one layers and adsorbed eAQ molecule are relaxed. There is 10 Å vacuum region between the two successive slabs to make sure the adsorbate (eAQ) can not contact with the subsequent slab. The adsorption energy is expressed as E a d s E P d (1 1 1 ) / P d M (1 1 1 ) e A Q E P d (1 1 1 ) / P d M (1 1 1 ) E e A Q 3 1 3 1 E P d (1 1 1 ) / P d
3M 1
(1 1 1 ) e A Q
, E P d (1 1 1 ) / P d
3M 1
(1 1 1 )
, and EeAQ are the total energy for the eAQ adsorbed
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where
(7)
on Pd(1 1 1) or Pd3M1(1 1 1) surface, the energy of free Pd(1 1 1) or Pd3M1(1 1 1), and the
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energy of free eAQ, respectively. 3. Results and discussion
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3.1. XRD results
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To confirm the structure of bimetallic monolithic catalysts after the addition of the
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second metal (La or Zn), XRD characterization of these monolithic catalyst was conducted.
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The results are shown in Fig. 1. For all of the monolithic catalysts, the characteristic peaks of
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COR correspond to the XRD standard PDF card no. 83-1385. The characteristic peaks at 2θ = 40.1 and 46.8° corresponding to Pd(111) and (200) crystal faces can be observed in the XRD
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pattern of Pd/SiO2/COR. However, these peaks for PdLa/SiO2/COR samples shift to smaller
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2θ values and for PdZn/SiO2/COR samples shift to larger 2θ values, which can be attributed to the formation of Pd-M alloy. This is consistent with previous studies on analogous Pd-M bimetallic catalysts [31,32]. In addition, the lattice constant calculated by DFT for Pd-Zn
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alloy Pd3Zn1 (3.92 Å) is smaller than pure Pd (3.94 Å), while for Pd3La1 (4.22 Å) is large than pure Pd (3.94 Å), which are consistent with the shifting of Pd(111) and Pd(200) peaks in XRD results. 3.2. H2-TPR results
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The reducibilities of Pd/SiO2/COR, La/SiO2/COR, Zn/SiO2/COR, and Pd-M/SiO2/COR were investigated by H2-TPR. The results are shown in Fig. 2. There is a reduction peak for the La/SiO2/COR catalyst, which is attributed to the reduction of La2O3 species. For Pd/SiO2/COR monometallic monolithic catalyst, there are no reduction peaks detected.
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However, there is a negative peak at 80 °C, which is attributed to the decomposition of PdH formed when the Pd species first interacts with hydrogen, indicating the Pd species can be
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easily reduced when they contact with hydrogen even at low temperature [33,34]. The metal particle size is crucial to the thermal stability of PdH, as reported previously in literatures
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[35-37]. Moreover, previous publications showed that the formed alloy in bimetallic catalysts
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after the addition of the second metal (Ag, Cu and Bi) suppresses the formation of PdH
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[38-40]. In this work, for the Pd-M/SiO2/COR bimetallic monolithic catalysts, there are not
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peaks of PdH detected, confirming the formation of Pd-M alloy. It can be seen that the
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reduction temperatures decrease with the increase of Pd loading, indicating Pd in Pd-M/SiO2/COR bimetallic monolithic catalysts can promote the reduction of the second
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metal species. This may be attributed to the hydrogen spillover from Pd to the second metal
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(M = La or Zn). It is noteworthy that the reduction temperature is remarkably influenced by the metal particle size. The reduction temperature of La/SiO2/COR is lower than that of Pd-La/SiO2/COR (M = L) bimetallic monolithic catalysts, which is attributed to the particle
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size of La/SiO2/COR being smaller than that of Pd-La/SiO2/COR bimetallic monolithic catalysts. 3.3. SEM, TEM, and HRTEM results SEM was used for observing the microscopic change of monolithic catalysts, while
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TEM was used for investigating the particle size distribution, providing evidence for the effect of the addition of the second metal on the surface metal distribution. The results are shown in Fig.3. SEM images of Pd-La/SiO2/COR monolithic catalyst show that the walls of Pd-La/SiO2/COR bimetallic monolithic catalyst are covered with a thin layer of SiO2
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washcoat [41,42]. The results from TEM show that the metal particles are well dispersed. The mean particle sizes of 1.2% Pd/SiO2/COR, 1.2% La/SiO2/COR, 0.8% Pd-0.4% La/SiO2/COR,
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0.6% Pd-0.6% La/SiO2/COR, and 0.4% Pd-0.8% La/SiO2/COR are 7.5, 2.9, 4.1, 3.5, and 3.1 nm, respectively, as calculated from 50 metal particles chosen from different regions. The
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particle size decreases with the increase of La loading, which is consistent with the H2-TPR
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results. The difference in surface energy between Pd (2.1 J m-2) and La (3.2 J m-2) results in
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the different particle sizes of Pd/SiO2/COR and La/SiO2/COR monometallic monolithic
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catalysts. That is, Pd diffuses faster than La, leading to the agglomeration of Pd [43]. Thus, a
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small amount addition of La to Pd/SiO2/COR can prevent the agglomeration of Pd. The higher selectivity and H2O2 yield obtained by 0.8% Pd-0.4% La/SiO2/COR and 0.6%
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Pd-0.6% La/SiO2/COR can be attributed to the addition of La leading to the better dispersion
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of Pd over these monolithic catalysts. Although the particle size of 0.4% Pd-0.8% La/SiO2/COR is smaller than those of 0.8% Pd-0.4% La/SiO2/COR and 0.6% Pd-0.6% La/SiO2/COR, the selectivity and H2O2 yield of 0.4% Pd-0.8% La/SiO2/COR is lower than
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those of 0.8% Pd-0.4% La/SiO2/COR and 0.6% Pd-0.6% La/SiO2/COR, which is due to the Pd active sites being covered by the large amount of La. The results from HRTEM (see Fig .4) show that the lattice plane spacing of 1.2% Pd/SiO2/COR, 0.8% Pd-0.4% La/SiO2/COR, and 0.8% Pd-0.4% Zn/SiO2/COR are 0.227, 0.232, and 0.224 nm, respectively, which is
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attributed to the (111) plane of Pd or Pd-M. This is due to the incorporation of the second metal into Pd, indicating the formation of Pd-M alloy. 3.4. XPS results To investigate the electronic effect in Pd-M/SiO2/COR after the addition of the second
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metal, the chemical states of Pd, La, and Zn were investigated by XPS analysis. The results are shown in Fig. 5. For Pd /SiO2/COR, there are two main peaks of Pd3d spectrum for Pd
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3d5/2 (335.3 eV) and Pd 3d3/2 (340.5 eV) of Pd0, which is consistent with the literatures [39,44]. For Pd-M/SiO2/COR, there is a shift of 0.2-1.0 eV toward to higher binding energy,
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which is assigned to the charge transfer from Pd to the second metal. For La/SiO2/COR, the
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binding energies of 836.7 eV and 851.5 eV for La 3d5/2 and La 3d3/2 correspond to the
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reported values for pure La [45]. For Pd-La/SiO2/COR, the binding energies of La 3d5/2 and
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La 3d3/2 shift to lower values, which is consistent with the Pd XPS data, confirming the
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electronic donation property of Pd in Pd-La/SiO2/COR bimetallic monolithic catalysts. For Zn/SiO2/COR, the binding energies of 1021.8 eV and 1045.1 eV for Zn 2p3/2 and Zn 2p1/2
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correspond to the reported values for pure Zn [20]. For Pd-Zn/SiO2/COR, the binding
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energies of Zn 2p3/2 and Zn 2p1/2 shift to lower values, which is consistent with the Zn XPS data, also confirming the electronic donation property of Pd in Pd-Zn/SiO2/COR bimetallic monolithic catalysts. The synergistic electronic effect, by which Pd donates electron density
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to the second metal, can promote the products to remove from the surface of monolithic catalysts, thus increasing the catalytic activity and selectivity. 3.5. H2–O2 titration The Pd dispersion and Pd-specific surface area of Pd/SiO2/COR and Pd-M/SiO2/COR
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were investigated by the H2–O2 titration [23], which are remarkably influenced by the second metal. The results are shown in Table 1. The Pd dispersion of Pd-La/SiO2/COR is higher than that of Pd-Zn/SiO2/COR, although the Pd loadings are identical. The results confirm that La is more conductive to improve the Pd dispersion of bimetallic monolithic catalysts and stop
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the Pd metal particles from migration and aggregation, indicating the stronger interaction between Pd and La, consistent with the TEM, H2-TPR, and XPS characterizations as
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mentioned above. Compared to Pd/SiO2/COR, the Pd dispersion and Pd-specific surface area
of Pd-M/SiO2/COR are higher. A small amount addition of La or Zn to Pd/SiO2/COR can
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prevent the agglomeration of Pd, consistent with the TEM characterization, indicating the
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strong synergistic effect between Pd and La or Zn.
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3.6. Catalytic hydrogenation of eAQ
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The catalytic performance of Pd/SiO2/COR monometallic and Pd-M/SiO2/COR
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bimetallic monolithic catalysts in the hydrogenation of eAQ was evaluated in the stainless steel fixed-bed reactor aforementioned. The results are shown in Table 1 and Fig. 6. The
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effect of the second metal (La or Zn) on the selectivity and H2O2 yield was investigated in the
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hydrogenation of eAQ. It was found that Pd-La/SiO2/COR has higher selectivity and H2O2 yield than Pd-Zn/SiO2/COR, while Pd/SiO2/COR has higher selectivity and H2O2 yield than La/SiO2/COR. The selectivity and H2O2 yield of Pd-La/SiO2/COR are higher than those of
A
Pd/SiO2/COR and La/SiO2/COR. Moreover, the selectivity and H2O2 yield of the mixture of Pd/SiO2/COR and La/SiO2/COR are lower than those of Pd-La/SiO2/COR, confirming the strong synergy between Pd and La in the bimetallic monolithic catalyst. The better catalytic performance of Pd-La/SiO2/COR is not only attributed to the synergy between Pd and the
16
second metal, as proved by the results of TEM, XPS, and TPR characterizations, but also attributed to the strong interaction between Pd3La1 (1 1 1) and the carbonyl group (C=O) of eAQ, as proved by DFT calculations. Since the metal loading plays an important role in the selectivity and H2O2 yield of
IP T
monometallic and bimetallic monolithic catalysts, the catalytic performance of a series of monolithic catalysts with various mass ratios of Pd/M (M =La or Zn) was investigated. The
SC R
results are shown in Fig. 6. It is noteworthy that the Pd-M/SiO2/COR bimetallic monolithic catalysts with the mass ratio of Pd/M = 2 achieve the highest selectivity and H2O2 yield,
U
which is attributed to the better Pd dispersion after the addition of La or Zn. The selectivity
N
and H2O2 yield decrease with the increase of mass ratio of Pd/M at mass ratio of Pd/M above
A
2, which may be due to the agglomeration of Pd. The selectivity and H2O2 yield increase with
M
the increase of mass ratio of Pd/M at mass ratio of Pd/M below 2, which is attributed to the
ED
better Pd dispersion after the addition of La or Zn. In addition, 0.8% Pd-0.4% M/SiO2/COR bimetallic monolithic catalysts has higher TOF
PT
than 1.2% Pd/SiO2/COR monometallic monolithic catalyst, which is due to the addition of
CC E
the second metal leading to better Pd dispersion. 3.7. DFT calculations on Pd(1 1 1) and Pd3M1(1 1 1) surfaces It is noteworthy that the hydrogenation of AQ over Pd pellet catalyst follows the
A
Langmuir-Hinshelwood (L-H) mechanism, in which the overall activation barrier of the hydrogenation of AQ is low (only 16.1 kcal mol−1) and the activation barrier for the decomposition of H2 on the top site of a Pd atom is also low (only 3.9 kcal·mol-1) [46]. Thus, the adsorption of eAQ over Pd or Pd-M surfaces of monolithic catalysts is vital to the eAQ
17
hydrogenation. In this work, the adsorption energies of eAQ over both Pd and PdM surfaces were calculated by DFT to investigate the difference of catalytic performance over Pd monometallic and Pd-M bimetallic monolithic catalysts. Herein, the two Pd3-La1 and Pd3-Zn1
IP T
crystal structures, along with pure Pd, were applied for modeling the Pd-M alloy. The results are shown in Fig. 7.
SC R
However, there are numerous possible configurations for the adsorption of eAQ over
Pd(1 1 1) and Pd3M1(1 1 1) surfaces. Thus, according to previous publications [46], the most
U
relevant structures of the adsorption of eAQ over these surfaces were selected. The most
N
stable and possible adsorption structures are shown in Fig. 8 and Fig. 9, respectively. The
A
adsorption energies (Eads) and bond lengths are given in Tables 2 and 3.
M
The most stable adsorption structure of eAQ over the optimized Pd(1 1 1) surface is that
ED
carbonyl oxygen atoms of eAQ and two benzene rings are located at bridge sites, in which the carbonyl bond is elongated by 0.048 when compared to single eAQ. That may be
PT
assigned to the interaction between the carbonyl π bond and the Pd(1 1 1) surface (i.e.,
CC E
electron back-donation), as confirmed in literatures [47]. The benzene rings of eAQ play an crucial role in promoting a flat eAQ adsorption on Pd(1 1 1) surface, which is consistent with the furan adsorption on Pd(1 1 1) by DFT calculations [48], indicating that the preferred
A
adsorption structure is with the benzene ring essentially parallel to the metal surface. However, the benzene rings have a strong affinity for Pd instead of the second metals [47]. Thus, the adsorption energy of eAQ over Pd(1 1 1) surface is 61.4 kcal·mol-1. The adsorption energies of eAQ over Pd3La1(1 1 1) and Pd3Zn1(1 1 1) decrease to 57.9 and 55.7 kcal·mol-1,
18
respectively. However, the adsorption energies of eAQ on Pd3La1(1 1 1) and Pd3Zn1(1 1 1) don’t change so much when compared to that on Pd(1 1 1), due to the strong bonding between carbonyl group and the second metal atoms. Compared to the adsorption of eAQ on Pd (1 1 1), the bond lengths (C=O, Pd-C/M-C, and Pd-O) (M = La and Zn) are elongated,
IP T
showing the weaker binding benzene rings and the stronger binding carbonyl group (see Tables 2 and 3). At the same time, the bond lengths of M-O (M =La and Zn) are shortened
SC R
apparently when compared to Pd-O, indicating the strong bonding between carbonyl group and the second metal atoms. Thus, the stronger adsorption of carbonyl group with the Pd-M
U
alloy surface results in the higher catalytic activity when compared to Pd/SiO2/COR
N
monometallic monolithic catalyst.
A
4. Conclusions
M
In this work, a series of Pd-La/SiO2/COR and Pd-Zn/SiO2/COR bimetallic monolithic
ED
catalysts were prepared by the co-impregnation method. The 0.8% Pd-0.4% La/SiO2/COR bimetallic monolithic catalyst presents the highest H2O2 yield (7.7 g·L-1) and selectivity (95%)
PT
compared to 1.2% Pd /SiO2/COR monometallic monolithic catalyst (4.1 g·L-1 and 60%). This
CC E
is attributed to the smaller metal particle and the strong interaction between Pd and the second metal after the addition of La, as proved by the characterization results of TEM, H2-TPR, and XPS.
A
Moreover, the adsorption of eAQ over Pd (1 1 1), Pd3La1(1 1 1) and Pd3Zn1(1 1 1) was
investigated by DFT calculations. It was found that the stronger adsorption between the Pd-La alloy surface and the carbonyl group of eAQ results in the higher catalytic activity. This work provides some theoretical insights into the reaction mechanism for eAQ
19
A
CC E
PT
ED
M
A
N
U
SC R
IP T
hydrogenation on Pd monometallic and Pd-M bimetallic monolithic catalysts.
20
Acknowledgement This work is financially supported by the Fundamental Research Funds for the Central Universities (XK1802-1).
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Table Captions
Table 1. Catalytic performance and textural parameters of different monolithic catalysts for the
IP T
hydrogenation of eAQ to eAQH2.
Table 2. Adsorption energies and optimized bond lengths of the most stable DFT optimized eAQ on Pd(1 1
SC R
1) and on Pd3M1(1 1 1) (M = La, and Zn) slabs.
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Table 3. Adsorption energies and optimized bond lengths of the optimized possible structures of eAQ on
A
CC E
PT
ED
M
A
N
Pd(1 1 1) and on Pd3M1(1 1 1) (M = La, and Zn) slabs.
25
I N U SC R
Table 1
Catalytic performance and textural parameters of different monolithic catalysts for the hydrogenation of eAQ to eAQH2.
Pd-loading (wt.%)
Second metal-loading (wt.%)
Selectivity (%)
Hydrogenation efficiency (g·L-1)
-
60
4.1
805
17
86
0.72
20
0.5
-
-
-
0.2
A
Washcoat
DPd (%)a
SPd (m2·g-1)
TOF （s-1）
30
La/SiO2/COR
ED
M
Sample
Washcoat loading (wt.%)
Pd efficiency (g H2O2·g-1 Pd·h-1)
SiO2
30
1.2
Pd-La/SiO2/COR
SiO2
30
0.8
0.4
95
7.7
1385
38
196
1.31
Pd-Zn/SiO2/COR
SiO2
30
0.8
0.4
86
6.5
950
31
157
1.1
SiO2
30
0.8
0.4
78
4.5
900
-
-
0.88
SiO2
1.2
A
CC E
PT
Pd/SiO2/COR
Pd/SiO2/COR+La/SiO2/COR
DPda was determined by H2-O2 titration analysis.
26
Table 2 Adsorption energies and optimized bond lengths of the most stable DFT optimized eAQ on Pd(1 1 1) and on Pd3M1(1 1 1) (M = Ni, Fe, Mn, and Cu) slabs.
C-O (Å)
Pd-Ca (Å)
Pd-Ob (Å)
M-Cc (Å)
M-Od (Å)
eAQ
-
1.237
-
-
-
-
eAQ/Pd(1 1 1)
61.4
1.285/1.286
2.276/2.268
2.382/2.372
-
-
eAQ/Pd3La1(1 1 1)
57.9
1.327/1.331
2.778/2.875
2.787
-
2.476
eAQ/Pd3Zn1(1 1 1)
55.7
1.297/1.275
2.349/2.397
-
2.699
SC R
U
N
2.387
A
CC E
PT
ED
M
A
Pd-Ca: the distance between C and the nearest Pd atom, as shown in Fig. 8. Pd-Ob: the distance between O and the nearest Pd atom, as shown in Fig. 8. M-Cc: the distance between C and the nearest second metal atom, as shown in Fig. 8. M-Od: the distance between O and the nearest second metal atom, as shown in Fig. 8.
27
IP T
Eads (kcal·mol-1)
model
Table 3 Adsorption energies and optimized bond lengths of the optimized possible structures of eAQ on Pd(1 1 1)
Pd-Ca (Å)
Pd-Ob (Å)
eAQ
-
1.237
-
-
eAQ/Pd(1 1 1)
61.4
1.285/1.286
2.276/2.268
eAQ/Pd3La1(1 1 1)-1
55.8
1.333/1.274
2.475/2.570
eAQ/Pd3La1(1 1 1)-2
53.5
1.330/1.333
3.141
eAQ/Pd3La1(1 1 1)-3
49.4
1.330/1.334
eAQ/Pd3Zn1(1 1 1)-1
53.9
1.299/1.276
eAQ/Pd3Zn1(1 1 1)-2
51.8
1.285/1.284
1.286/1.285
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CC E A
-
2.945
-
2.310
3.080/3.437
2.436
-
3.014/3.194
2.522
2.056
2.009/2.334
2.379
-
2.385/2.465
2.577
-
2.477
2.542
2.475/2.698
2.678
-
U
-
3.141
Pd-Ca: the distance between C and the nearest Pd atom, as shown in Fig. 9. Pd-Ob: the distance between O and the nearest Pd atom, as shown in Fig. 9. M-Cc: the distance between C and the nearest second metal atom, as shown in Fig. 9. M-Od: the distance between O and the nearest second metal atom, as shown in Fig. 9.
28
-
N
47.3
M-Od (Å)
2.382/2.372
A
ED
eAQ/Pd3Zn1(1 1 1)-3
M-Cc (Å)
IP T
C-O (Å)
model
SC R
Eads (kcal·mol-1)
M
and on Pd3M1(1 1 1) (M = Ni, Fe, Mn, and Cu) slabs.
Figure Captions Fig. 1. XRD patterns of the monolithic catalysts. (a), 1.2% Pd/SiO2/COR; (b), 1.2% La/SiO2/COR; (c), 1.2% Zn/SiO2/COR; (d), 0.8% Pd-0.4% La/SiO2/COR; (e), 0.6% Pd-0.6% La/SiO2/COR; (f), 0.4% Pd-0.8% La/SiO2/COR; (g), 0.8% Pd-0.4%
Fig. 2. H2-TPR spectra of different monolithic catalysts.
SC R
IP T
Zn/SiO2/COR; (h), 0.6% Pd-0.6% Zn/SiO2/COR; (i), 0.4% Pd-0.8% Zn/SiO2/COR.
(A) Pd-La/SiO2/COR. (a) 1.2% Pd/SiO2/COR; (b) 1.2% La/SiO2/COR; (c) 0.8% Pd-0.4% La /SiO2/COR;
U
(d) 0.6% Pd-0.6% La /SiO2/COR; (e) 0.4% Pd-0.8% La /SiO2/COR.
N
(B) Pd-Zn/SiO2/COR. (a) 1.2% Pd/SiO2/COR; (b) 1.2% Zn/SiO2/COR; (c) 0.8% Pd-0.4% Zn/SiO2/COR;
M
A
(d) 0.6% Pd-0.6% Zn/SiO2/COR; (e) 0.4% Pd-0.8% Zn/SiO2/COR.
ED
Fig. 3. SEM images of 0.8% Pd-0.4% La/SiO2/COR catalyst (a, b); TEM images of 1.2% Pd/SiO2/COR (c, d); 1.2% La/SiO2/COR (e, f); 0.8% Pd-0.4% La/SiO2/COR (g, h); 0.6% Pd-0.6% La/SiO2/COR (i, j); and
PT
0.4% Pd-0.8% La /SiO2/COR (k, l). Pd particle size distributions of 1.2% Pd/SiO2/COR (m); 1.2%
CC E
La/SiO2/COR (n); 0.8% Pd-0.4% La/SiO2/COR (o); 0.6% Pd-0.6% La/SiO2/COR (p); and 0.4% Pd-0.8% La /SiO2/COR (q).
A
Fig. 4. HRTEM images of 1.2% Pd/SiO2/COR (a); 0.8% Pd-0.4% La/SiO2/COR (b); 0.8% Pd-0.4% Zn/SiO2/COR (c).
Fig. 5. Pd3d, La3d, and Zn2p spectra of different monolithic catalysts.
29
(A) Pd3d (a) 1.2% Pd/SiO2/COR; (b) 0.8% Pd-0.4% La/SiO2/COR; (c) 0.6% Pd-0.6% La/SiO2/COR; (d) 0.4% Pd-0.8% La/SiO2/COR. (B) La3d (a) 1.2% La/SiO2/COR; (b) 0.8% Pd-0.4% La/SiO2/COR; (c) 0.6% Pd-0.6% La/SiO2/COR; (d) 0.4% Pd-0.8% La/SiO2/COR.
IP T
(C) Pd3d (a) 1.2% Pd/SiO2/COR; (b) 0.8% Pd-0.4% Zn/SiO2/COR; (c) 0.6% Pd-0.6% Zn/SiO2/COR; (d) 0.4% Pd-0.8% Zn/SiO2/COR.
SC R
(D) Zn2p (a) 1.2% Zn/SiO2/COR; (b) 0.8% Pd-0.4% Zn/SiO2/COR; (c) 0.6% Pd-0.6% Zn/SiO2/COR; (d)
U
0.4% Pd-0.8% Zn/SiO2/COR.
N
Fig. 6. Effect of the second metal loadings on selectivity (a) and H2O2 yield (b).
A
Reaction conditions: pressure = 101 kPa, temperature = 80°C, concentration of eAQ solution = 60 g·L-1,
M
flow rate of eAQ solution = 10 mL·min-1, and flow rate of H2 gas = 100 mL·min-1.
ED
A, 1.2% Pd/SiO2/COR; B, 0.96% Pd-0.24% M/SiO2/COR; C, 0.8% Pd-0.4% M/SiO2/COR; D, 0.6%
PT
Pd-0.6% M/SiO2/COR; E, 0.4% Pd-0.8%. M/SiO2/COR; F, 0.24% Pd-0.96% M/SiO2/COR.
CC E
Fig. 7. Bulk monometallic Pd and bimetallic Pd-M (M = La and Zn) alloy structures.
Fig. 8. The most stable DFT optimized structures of eAQ (a) and eAQ adsorbed on Pd(1 1 1) (b), Pd3La1(1
A
1 1) (c), and Pd3Zn1(1 1 1) (d).
Fig. 9. The possible structures optimized by DFT of eAQ (a) and eAQ adsorbed on Pd3La1(1 1 1)-1 (b), Pd3La1(1 1 1)-2 (c), Pd3La1(1 1 1)-3 (d), Pd3Zn1(1 1 1)-1 (e), Pd3Zn1(1 1 1)-2 (f), and Pd3Zn1(1 1 1)-3 (g).
30
(i)
● cordierite
●
● ●●
●● ●
●
(i)
intensity
(h) (g) (f) (e)
IP T
(d) (c) (b)
10
20
30
40
2θ (degree)
● cordierite ▼ Pd ★ PdLa
★ ▼
42
44
46
U
◆
◆
◆
●
▼
(a)
48
40
50
2θ (degree)
42
44
46
(i)
◆
(h)
◆
(g)
▼
(a)
(d)
ED
▼
(200)
●
A
(e)
★
★
◆
(f)
intensity
●
★
★
40
(111)
M
intensity
●
●
★
● cordierite ▼ Pd ◆ PdZn
(iii)
(200)
(111)
50
N
(ii)
SC R
(a)
48
50
CC E
PT
2θ (degree)
Fig. 1. XRD patterns of the monolithic catalysts. (a), 1.2% Pd/SiO2/COR; (b), 1.2% La/SiO2/COR; (c), 1.2% Zn/SiO2/COR; (d), 0.8% Pd-0.4%
A
La/SiO2/COR; (e), 0.6% Pd-0.6% La/SiO2/COR; (f), 0.4% Pd-0.8% La/SiO2/COR; (g), 0.8% Pd-0.4% Zn/SiO2/COR; (h), 0.6% Pd-0.6% Zn/SiO2/COR; (i), 0.4% Pd-0.8% Zn/SiO2/COR.
31
476
(A)
(e) 442
(d)
387
Intensity
(c) 355
(a)
400
600
T (C)
576
(B)
800
SC R
200
50
IP T
(b)
U
(e)
N
443
200
400
(d) (c) (b) (a)
600
800
T (C)
CC E
PT
50
ED
M
324
A
Intensity
381
Fig. 2. H2-TPR spectra of different monolithic catalysts. (A) Pd-La/SiO2/COR. (a) 1.2% Pd/SiO2/COR; (b) 1.2% La/SiO2/COR; (c) 0.8% Pd-0.4% La /SiO2/COR;
A
(d) 0.6% Pd-0.6% La /SiO2/COR; (e) 0.4% Pd-0.8% La /SiO2/COR. (B) Pd-Zn/SiO2/COR. (a) 1.2% Pd/SiO2/COR; (b) 1.2% Zn/SiO2/COR; (c) 0.8% Pd-0.4% Zn/SiO2/COR; (d) 0.6% Pd-0.6% Zn/SiO2/COR; (e) 0.4% Pd-0.8% Zn/SiO2/COR.
32
IP T
(b)
M
A
N
U
SC R
(a)
50 nm
20 nm (d)
A
CC E
PT
ED
(c)
10 nm
50 nm (e)
(f)
33
IP T
50 nm
20 nm (h)
M
A
N
U
SC R
(g)
10 nm
ED
50 nm (j)
A
CC E
PT
(i)
50 nm
10 nm
(k)
(l)
34
70
35
dm = 2.9 nm
30
60
25
50
counts (%)
20 15
40 30
10
20
5
10 0
0 2
4
6
8
10
12
2
14
4
8
10
SC R
45
70
dm = 3.5nm
40
dm = 4.1 nm
60
6
Diameter (nm) (n)
Diameter (nm) (m)
30
25 20 15
20
10
A
5
10
U
40
30
N
counts (%)
35
50
counts (%)
IP T
counts (%)
dm = 7.5 nm
0
0 4
6
8
Diameter (nm) (o)
2
4
6
8
10
Diameter (nm) (p)
dm =3.1 nm
ED
60
10
M
2
55 45 40 35
PT
counts (%)
50
30 25
A
CC E
20 15 10
5 0 2
4
6
8
10
Diameter (nm) (q)
Fig. 3. SEM images of 0.8% Pd-0.4% La/SiO2/COR catalyst (a), (b); TEM images of 1.2% Pd/SiO2/COR (c), (d); 1.2% La/SiO2/COR (e), (f); 0.8% Pd-0.4% La/SiO2/COR (g), (h); 0.6% Pd-0.6% La/SiO2/COR (i), (j); and 0.4% Pd-0.8% La /SiO2/COR (k), (l). Pd particle size distributions of 1.2% Pd/SiO2/COR (m); 1.2% La/SiO2/COR (n); 0.8% Pd-0.4% La/SiO2/COR (o); 0.6% Pd-0.6% La/SiO2/COR (p); and 0.4% Pd-0.8% La /SiO2/COR (q). 35
d = 0.227 nm
d=0.235 nm
IP T
d=0.224 nm
5 nm
5 nm (b)
(c)
SC R
(a)
Fig. 4 HRTEM images of 1.2% Pd/SiO2/COR (a); 0.8% Pd-0.4% La/SiO2/COR (b); 0.8% Pd-0.4%
A
CC E
PT
ED
M
A
N
U
Zn/SiO2/COR (c).
36
Pd3d3/2
Pd3d5/2
(B)
(A) 340.5
851.5
836.7
(a)
(a) (b)
Intensity
Intensity
335.3
La3d3/2
La3d5/2
(b)
(c)
(c)
330
332
334
336
338
340
342
344
830
835
840
Binding energy (eV)
Zn2p3/2
Pd3d3/2
(C)
(D) 340.5
336
338
340
342
U
(a) (b) (c)
344
1015
1020
(d) 1025
1030
1035
1040
1045
1050
Binding energy (eV)
ED
Binding energy (eV)
M
(d)
334
1045.1
N
(c)
332
855
Zn2p1/2
A
Intensity
(b)
330
1021.8
(a)
Intensity
335.3
850
Binding energy (eV)
SC R
Pd3d5/2
845
IP T
(d) (d)
PT
Fig. 5. Pd3d, La3d, and Zn2p spectra of different monolithic catalysts. (A) Pd3d (a) 1.2% Pd/SiO2/COR; (b) 0.8% Pd-0.4% La/SiO2/COR; (c) 0.6% Pd-0.6% La/SiO2/COR; (d)
CC E
0.4% Pd-0.8% La/SiO2/COR.
(B) La3d (a) 1.2% La/SiO2/COR; (b) 0.8% Pd-0.4% La/SiO2/COR; (c) 0.6% Pd-0.6% La/SiO2/COR; (d) 0.4% Pd-0.8% La/SiO2/COR.
A
(C) Pd3d (a) 1.2% Pd/SiO2/COR; (b) 0.8% Pd-0.4% Zn/SiO2/COR; (c) 0.6% Pd-0.6% Zn/SiO2/COR; (d) 0.4% Pd-0.8% Zn/SiO2/COR. (D) Zn2p (a) 1.2% Zn/SiO2/COR; (b) 0.8% Pd-0.4% Zn/SiO2/COR; (c) 0.6% Pd-0.6% Zn/SiO2/COR; (d) 0.4% Pd-0.8% Zn/SiO2/COR.
37
100
60
40
IP T
Selectivity (%)
80
PdLa PdZn Pd
0
A
B
C
D
E
7
PdLa PdZn Pd
N
8
A M
5 4 3
1
PT
0
ED
H2O2 yield (g·L-1)
6
2
F
U
Catalysts
SC R
20
B
C
D
E
F
Catalysts
CC E
A
Fig. 6. Effect of the second metal loadings on selectivity (a) and H2O2 yield (b). Reaction conditions: pressure = 101 kPa, temperature = 80°C, concentration of eAQ solution = 60 g·L-1,
A
flow rate of eAQ solution = 10 mL·min-1, and flow rate of H2 gas = 100 mL·min-1. A, 1.2% Pd/SiO2/COR; B, 0.96% Pd-0.24% M/SiO2/COR; C, 0.8% Pd-0.4% M/SiO2/COR; D, 0.6% Pd-0.6% M/SiO2/COR; E, 0.4% Pd-0.8%. M/SiO2/COR; F, 0.24% Pd-0.96% M/SiO2/COR.
38
SC R
IP T
Pd
Pd3Zn1
ED
M
A
N
U
Pd3La1
CC E
PT
Pd
La
A
Fig. 7. Bulk monometallic Pd and bimetallic Pd-M (M = La and Zn) alloy structures.
39
Zn
(a)
IP T
1.237
SC R
(b) 2.382
1.285
U
2.276
2.268
2.372
A
N
1.286
ED
M
Eads = 61.4 kcal·mol-1
2.787
PT
(c)
1.327
A
CC E
2.778 1.331
2.875 2.476
Eads = 57.9 kcal·mol-1
40
(d) 2.387 1.297 2.349 2.397 1.275
IP T
2.699
O
H
Pd
La
Zn
M
A
N
U
C
SC R
Eads = 55.7 kcal·mol-1
ED
Fig. 8. The most stable DFT optimized structures of eAQ (a) and eAQ adsorbed on Pd(1 1 1) (b), Pd3La1(1
A
CC E
PT
1 1) (c), and Pd3Zn1(1 1 1) (d).
41
(a)
SC R
IP T
1.237
(b)
1.333
2.475 1.274
A
N
U
2.310
M
2.945
Eads = 55.8 kcal·mol-1
CC E
(c)
PT
ED
Pd3La1(1 1 1)-1
2.570
3.080 1.330 3.141
A
2.436 1.333 3.437
Pd3La1(1 1 1)-2
Eads = 53.5 kcal·mol-1
42
(d)
3.014 1.330 2.522
Pd3La1(1 1 1)-3
Eads = 49.4 kcal·mol-1
2.009
1.299 2.056 2.379 1.276 2.334
ED
M
A
N
U
(e)
SC R
3.194
CC E
(f)
Eads = 53.9 kcal·mol-1
PT
Pd3Zn1(1 1 1)-1
2.447 1.285 2.385
A
2.465 1.284 2.577
Pd3Zn1(1 1 1)-2
Eads = 51.8 kcal·mol-1
43
IP T
3.141 1.334
(g) 2.475
1.286
2.678 2.542 2.698
Pd3Zn1(1 1 1)-3
O
H
Pd
La
Zn
M
A
N
C
U
SC R
Eads = 47.3 kcal·mol-1
IP T
1.285
ED
Fig. 9. The possible structures optimized by DFT of eAQ (a) and eAQ adsorbed on Pd3La1(1 1 1)-1 (b),
A
CC E
PT
Pd3La1(1 1 1)-2 (c), Pd3La1(1 1 1)-3 (d), Pd3Zn1(1 1 1)-1 (e), Pd3Zn1(1 1 1)-2 (f), and Pd3Zn1(1 1 1)-3 (g).
44
S0255-2701(18)30342-8 https://doi.org/10.1016/j.cep.2018.11.006 CEP 7421
To appear in:
Chemical Engineering and Processing
Received date: Revised date: Accepted date:
21 March 2018 30 October 2018 12 November 2018
Please cite this article as: Guo Y, Dai C, Lei Z, Hydrogenation of 2ethylanthraquinone on Pd-La/SiO2 /cordierite and Pd-Zn/SiO2 /cordierite bimetallic monolithic catalysts, Chemical Engineering and Processing - Process Intensification (2018), https://doi.org/10.1016/j.cep.2018.11.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hydrogenation of 2-ethylanthraquinone on Pd-La/SiO2/cordierite and Pd-Zn/SiO2/cordierite bimetallic monolithic catalysts Yanyan Guoa, Chengna Daib, Zhigang Leib,* a
*Corresponding
IP T
Department of Medicinal Chemistry, School of Pharmacy, Anhui University of Chinese Medicine, Hefei 230031, China bState Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Energy Environmental Catalysis, Beijing University of Chemical Technology, Box 266, Beijing 100029, China
author. Tel: +86 10 64433695.
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E-mail address: [email protected] (Z. Lei).
Graphical abstract
and selectivity (95%) for the hydrogenation of 2-ethylanthraquinone with
N
g·L-1)
U
0.8% Pd-0.4% La/SiO2/cordierite monolithic catalyst exhibits the highest H2O2 yield (7.7
M
A
anthraquinone method.
eAQH2
C
O
H
A
CC E
PT
ED
eAQ
H2
1
Pd
La
Highlights
A series of Pd-La(Zn)/SiO2/cordierite bimetallic monolithic catalysts were prepared;
0.8%Pd-0.4%La/SiO2/COR catalyst is superior to 1.2%Pd/SiO2/COR due to
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synergistic effect; The mechanism of eAQ-PdM (M = La and Zn) surface interactions was explored by
U
SC R
DFT calculations.
N
Abstract A series of Pd-La/SiO2/cordierite (cordierite = COR) and Pd-Zn/SiO2/COR
A
bimetallic monolithic catalysts were prepared, characterized, and tested for the hydrogenation
M
of 2-ethylanthraquinone (eAQ) as the key step of anthraquinone method for production of
ED
H2O2. It was found that the metal particle size is remarkably dependent on the Pd/M (M = La, and Zn) mass ratio of bimetallic monolithic catalysts. By virtue of its small metal particle size
PT
and the strong interaction, the 0.8% Pd-0.4% La/SiO2/COR monolithic catalyst presents the
CC E
highest H2O2 yield (7.7 g·L-1) and selectivity toward active quinones (95%), which is attributed to the addition of La leading to the synergistic effect between Pd and La. Moreover, the density functional theory (DFT) calculations for the adsorption of eAQ over the surface of
A
Pd-La/SiO2/COR and Pd-Zn/SiO2/COR bimetallic monolithic catalysts have been conducted to further understand the mechanism of molecule–surface interactions between eAQ and the PdM(1 1 1) or Pd(1 1 1) surfaces. Detailed studies showed that the high catalytic activity of Pd-La/SiO2/COR bimetallic monolithic catalyst is attributed to the stronger adsorption
2
between Pd3La1 (1 1 1) and the carbonyl group of eAQ. Keywords: Bimetallic monolithic catalysts; eAQ hydrogenation; DFT calculation; Synergistic effect
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1. Introduction Hydrogen peroxide (H2O2) is an important green chemical raw material and
SC R
environmental friendly oxidizing agent in many industry areas [1-4]. Up to now, more than
98% of H2O2 is manufactured by the anthraquinone process. In this cyclic process, the (eAQ)
is
first
hydrogenated
to
the
U
2-ethyl-9,10-anthraquinone
N
2-ethyl-9,10-anthrahydroquinone (eAQH2), and then the eAQH2 is oxidized to produce eAQ
products,
but
2-ethyl-anthrone
(eAN),
M
desired
A
and H2O2 (see Scheme 1). In the catalytic hydrogenation of eAQ, eAQH2 and H4eAQH2 are 2-ethyloxoanthrone
(OXO),
ED
2-ethyl-tetrahydro-anthrone (H4eAN), and octahydroanthrahydroquinone (H8eAQH2) are undesired products, which are attributed to the deep hydrogenation of eAQH2 [5]. Therefore,
CC E
PT
eAQ and 2-ethyl-5,6,7,8-tetrahydro-9,10-anthraquinone (H4eAQ) are active quinones.
A
Reactions to produce H2O2
O Et
+ O
H2O2
eAQ H2
O2 OH
OH Et
OH
H2
Et
OH
eAQH2
H4eAQH2 H2 OH
O
Et
Et
H
OH
OH
OXO H2
H8eAQH2 3
O
These four products can not produce H2O2
Et
eAN
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Scheme 1. The reaction route of the anthraquinone hydrogenation/oxidation process to produce H2O2 [6].
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Supported Pd catalysts are the most commonly used catalysts in the hydrogenation of eAQ, which is attributed to the supported Pd catalysts presenting better catalytic activities
U
than other pellet catalysts in the hydrogenation of eAQ. However, mass transfer is the
N
controlling factor in the catalytic hydrogenation of eAQ [7-11]. In principle, the internal and
A
external diffusion resistances decrease when using monolithic catalysts, and thus the overall
M
reaction rate can be improved. Thus, the supported Pd monolithic catalysts were investigated
ED
in the hydrogenation of eAQ in previous publications. Moreover, the studies are mainly focused on modifying the supports of monolithic catalysts to improve the catalytic activities
PT
[12-14]. For example, the Pd/Al2O3/COR has higher selectivity and H2O2 yield than the
CC E
Pd/Al2O3 pellet catalyst in the catalytic hydrogenation of eAQ, due to the monolithic catalyst having short diffusion distances [12]. The Pd/SiO2/COR monolithic catalyst has higher catalytic stability than the Pd/Al2O3/COR monolithic catalyst in the catalytic hydrogenation
A
of eAQ, due to the Pd/SiO2/COR monolithic catalyst having more regular structure and weaker acidity [13]. In our previous work, the Pd/SiO2/COR monolithic catalyst have higher selectivity and H2O2 yield than Pd/Al2O3 pellet catalyst, due to the high mass transfer coefficient [6,14].
4
In addition to modifying the support, the modification of the active sites for the catalysts (i.e., morphology and composition) can improve the catalytic performance. For example, the catalytic activity and selectivity of Pd-Fe/SiO2 bimetallic catalyst were higher than those of Pd/SiO2 catalysts in the Liquid-phase hydrogenation of phenylacetylene to styrene [15].
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Pd-Cu bimetallic catalyst presents higher conversion of furfural and higher selectivity (> 98%) than Pd/SiO2 catalyst [16]. The higher catalytic performance of such hybrid materials is due
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to the synergistic inter-metallic interactions between Pd and the second metal [17-19]. Recently, the combination of Pd with La or Zn has attracted particular attention. For example,
U
the catalytic activity and selectivity of Pd-Zn/TiO2 and Pd-Zn/CeO2 catalysts are higher than
N
those of Pd/TiO2 in semi-hydrogenation of acetylene due to the synergistic interactions
A
between Pd and Zn of PdZn alloy [20,21]. Pd-La/spinel was tested for the preparation of
M
2,6-diisopropylaniline. It showed that the activity and selectivity of Pd-La/spinel increase
ED
when compared to Pd catalyst through neutralizing the strong acid sites on the surface of catalyst and decreasing the formation of carbon [22]. Therefore, the synergistic inter-metallic
PT
interactions between Pd and La or Zn over Pd-based bimetallic monolithic catalysts offer the
CC E
promise for simultaneously increasing both selectivity and H2O2 yields in the hydrogenation of eAQ.
Therefore, the present work focuses on improving the H2O2 yields and selectivity in the
A
hydrogenation of eAQ by adding the La, or Zn into Pd/SiO2/COR monolithic catalyst. In this work, a series of Pd, La, and Zn monometallic, and Pd-M (M = La or Zn) monolithic catalysts were prepared with the co-impregnation method for the catalytic hydrogenation of eAQ. Then, transmission electron microscopy (TEM), scanning electron microscopy (SEM),
5
X-ray photoelectron spectroscopy (XPS), H2 temperature programmed reduction (H2-TPR), and H2–O2 titration were conducted to investigate the structure and composition of monolithic catalysts, thus revealing the synergistic effect between Pd and the second metal (M= La, Zn). Finally, a deep mechanistic understanding for the hydrogenation of eAQ on
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different monometallic and bimetallic monolithic catalysts was performed by DFT calculations.
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2. Experimental section 2.1. Materials
U
Cordierite was purchased from Nanning Yilaite Environmental Protection Technology
N
Co. PdCl2 (99.5 wt%) was purchased from Tianjin Guangfu Fine Chemical Research Institute.
A
C4H6O4Zn·2H2O (99 wt%) and LaCl3·7H2O (99.9 wt%) were purchased from Shanghai
M
Aladdin Chemistry Co. SiO2 sol (30 wt%) was purchased from Sigma Aldrich (Shanghai)
2.2. Catalyst preparation
ED
trading Co., Ltd.
PT
2.2.1. Washcoating of monoliths
CC E
Because the surface area of bare monoliths is low, they can’t provide sufficient active sites for active components. Thus, the monoliths were coated a layer of SiO2 sol by impregnation method to increase the surface area of monolith supports. The cylindrical
A
cordierite ( 132× 70 mm and 400 channels per square inch (cpsi)) were cut into pieces of monolithic supports ( 20 × 10 mm). The supports were heated in 15 wt.% nitric acid for 4 h at 80 °C, and then washed by the distilled water to neutrality. Afterward, the supports pretreated with nitric acid were dried in an oven at 100 °C for 2 h, and then calcined in a
6
muffle furnace at 550 °C for 4 h. After acid pretreatment, the monolith supports were dipped into 30 wt% SiO2 sol solution for 5 min, and then the coated monolith supports were dried in an oven for 15 min at 100°C. Before the desired loading of monolith supports were obtained, the steps were repeated several times. Finally, the monolith supports with the desired loading
IP T
were calcined in a muffle furnace at 550 °C for 4 h. 2.2.2. Metal deposition
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A series of Pd, La, and Zn monometallic, and Pd-M bimetallic monolithic catalysts were
prepared by co-impregnation method. The coated monolith supports were impregnated into
U
the PdCl2, LaCl3, and Zn(CH3COOH)2 aqueous solutions at 100 °C for 24 h to obtain the
N
precursors of Pd/SiO2/COR, La/SiO2/COR, and Zn/SiO2/COR. The PdCl2 and LaCl3 aqueous
A
solutions were mixed, and the mixed solution was stirred for 30 min. Meanwhile, the PdCl2
M
and Zn(CH3COOH)2 aqueous solutions were mixed, and the mixed solution was stirred for 30
ED
min. Then, the coated monolith supports were impregnated into the two mixed solutions at 100 °C for 24 h to obtain the precursors of Pd-M bimetallic monolithic catalysts. The
PT
precursors Pd/SiO2/COR, La/SiO2/COR, Zn/SiO2/COR monometallic and Pd-M/SiO2/COR
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bimetallic monolithic catalysts were calcined in an muffle furnace at 550 °C for 4 h to obtain the oxide species of Pd/SiO2/COR, La/SiO2/COR, Zn/SiO2/COR monometallic and Pd-M/SiO2/COR bimetallic monolithic catalysts. Prior to the catalytic experiment, these
A
oxide species of bimetallic monolithic catalysts were reduced by H2 gas to obtain the Pd/SiO2/COR, La/SiO2/COR, Zn/SiO2/COR monometallic and Pd-M/SiO2/COR bimetallic monolithic catalysts. Herein, the loading amount of SiO2 washcoat was calculated based on all of the
7
monolithic catalyst weight including COR and SiO2, while the loading amount of the metal (Pd, La or Zn) analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) was calculated based on the weight of SiO2 washcoat. 2.3. Catalyst characterization
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The washcoat of monolithic catalysts was observed by scanning electron microscopy (SEM) (JSM-6701F, JEOL, Japan), which was operated at an accelerating voltage of 5.0 kV.
ion sputtering instrument to make the samples conductive.
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Prior to observation, the surface of the samples was sprayed a thin layer of gold film using an
U
Transmission electron microscopy (TEM) (JEM-2000 EX, JEOL, Japan), conducted at
N
an accelerating voltage of 120 kV, was used to measure the particle size and morphology of
A
monolithic catalysts. Prior to observation, the samples of monolithic catalysts were ground
M
into powder and then dispersed uniformly in the ethanol solution by ultrasonication. Drops of
ED
the suspension were dipped onto a transparent carbon foil supported by copper grid and dried in air.
PT
Hydrogen temperature programmed reduction (H2-TPR) curves of monolithic catalysts
CC E
were measured to investigate the reducibilities of monolithic catalysts using a Thermo Electron TPD/R/O 1100 series instrument equipped with a thermal conductivity detector (TCD).
A
The composition and chemical state of active species on the surface of monolithic
catalysts were measured by X-ray photoelectron spectroscopy (XPS) spectra by an photoelectron spectrometer (ESCALAB 250, thermoFisher Scientific, USA) using C as the internal standard (C1s = 284.6 eV).
8
ICP-AES (ICP-7500, Shimadzu Corporation, Japan) was used to measure the actual Pd and the second metal contents of monolithic catalysts. To investigate the Pd dispersion degree and Pd surface area of Pd-M/SiO2/COR bimetallic monolithic catalysts after the addition of the second metal (La or Zn), the H2–O2
IP T
titration was performed on a Thermo Electron TPD/R/O 1100 series instrument, which was equipped with a thermal conductivity detector (TCD). The details on operating procedure can
were calculated by [23]:
S Pd ( m g
1
)
100
U
3 2 2 .4 W P
2 VH N 10 3 2 2 .4 W P
(1)
-3
100
(2)
A
2
-3
N
D P d (% )
2 VH M 10
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be found in our previous work [6]. The Pd dispersion (DPd) and Pd metal surface area (SPd)
M
where VH is the volume of H2 used for the titration of O2 (mL); M is the relative molecular
ED
weight of Pd (g·mol-1); W is the catalyst weight (g); P is the mass fraction of Pd; N (= 6.023 × 1023) is Avogadro’s constant; and σ is the cross-sectional area of a Pd atom (8.97 × 10−20 m2).
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2.4. Catalytic activity test
CC E
The catalytic performance of Pd/SiO2/COR, La/SiO2/COR, Zn/SiO2/COR, and Pd-M/SiO2/COR monolithic catalysts were tested in the catalytic hydrogenation of eAQ, performed in a stainless steel fixed-bed reactor (20 mm in inner diameter and 450 mm in
A
height). Prior to reaction, 3.6 g eAQ was dissolved in 60 mL working solution, which is composed of 15 mL trioctyl phosphate and 45 mL industrial grade C9 aromatics, to obtain the new working solution. Ten pieces of monolithic catalysts ( 20 × 10 mm) were fixed in the reaction zone of fixed-bed reactor. The precursors of monolithic catalysts were first reduced
9
at a certain temperature for 3 h to obtain the monolithic catalysts. Then, the catalytic hydrogenation of eAQ was performed at 80 °C, while the H2 gas (99.999 wt%) of 100 mL·min-1 and the working solution of 10 mL·min-1 were fed into the fixed-bed reactor. The details on operating parameters can be found in our previous work [6].
IP T
After hydrogenation, 5 mL of the hydrogenated working solution was placed into the oxidation device, and oxidized at 25 °C and 101 kPa for 30 min with the air produced from
SC R
air generator (GC-ready SPB-5000 Automatic Air Source, China). Then, the working solution
was extracted by distilled water to obtain H2O2 solution. Prior to the titration, the 15 mL
U
sulfuric acid solution (25 wt%) were placed into the H2O2 solution. Finally, the amount of
N
H2O2 was titrated using the potassium permanganate solution.
A
The H2O2 yield (B), catalyst selectivity (S), and turnover frequency (TOF) were
V
5C
K M nO 4
ED
B
M
calculated by
S
n eA Q n H
4
eA Q
PT
H 2O 2
(3)
2V 1 0 0 %
n0 (eA Q )
CC E
M
K M nO 4
nH
2O 2
1 0 0 %
(4)
n0 (eA Q )
TOF
n0C
(5)
n cat t
n cat
WP M
D Pd
(6)
Pd
A
where CKMnO4 is the concentration of KMnO4 solution (mol·L-1); and VKMnO4 and V represent the volumes of KMnO4 solution and H2O2 solution (mL), respectively; S is the selectivity toward active quinones (eAQ and H4eAQ); n0 and n are the molar concentrations of components in the initial working solution and oxidized solution (mol·L-1), respectively;
10
nH
is the molar concentration of component of H2O2 in the oxidized solution (mol·L-1). C is
2O 2
the conversion of eAQ at the reaction time t; ncat is the mole number of exposed Pd atom; MPd is the molar mass of Pd; W is the catalyst weight (g); P is the mass fraction of Pd; and DPd is the Pd dispersion.
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2.5. Computational method Density functional theory (DFT) calculations, based on the first-principles, were
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performed to investigate the optimized surface structures of catalysts and the adsorption energy for adsorbate on the optimized surface of catalysts [24,25]. In this work,
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spin-polarized periodic DFT calculations were conducted using the Vienna Ab-Initio
N
Simulation Package (VASP). The valence electrons with a cutoff energy of 400 eV were
A
described by plane wave basis sets, and the core electrons were replaced by the projector
M
augmented wave (PAW) pseudo-potentials [26,27]. The exchange–correlation functional was
ED
described using the generalized gradient approximation (GGA), which was proposed by Perdew, Burke, and Ernzerhof (PBE) [28]. The sampling of the Brillouin-zone for p(3 × 2)
PT
lateral supercell was restricted to a 2 × 2 × 1 Monkhorst-Pack grid. The criterion of
CC E
convergence is less than 0.05 eV·Å−1 and 4 × 10−6 eV for the force and energy differences, respectively.
In the calculation, the Pd3M1 (M = La or Zn), along with pure Pd, was applied for
A
modeling the Pd3M1 alloys. The optimized lattice constants are a = b = c = 3.94 Å for Pd, a = b = c = 4.22 Å for Pd3La1, and a = b = c = 3.92 Å for Pd3Zn1, consistent with the values reported in literature [29,30]. The closed packed (3 2) Pd(111), Pd3La1(111) and Pd3Zn1(111) surfaces were modeled using a 3-layer Pd slab, in which the atoms in the bottom two layers
11
are fixed, while the atoms in the top one layers and adsorbed eAQ molecule are relaxed. There is 10 Å vacuum region between the two successive slabs to make sure the adsorbate (eAQ) can not contact with the subsequent slab. The adsorption energy is expressed as E a d s E P d (1 1 1 ) / P d M (1 1 1 ) e A Q E P d (1 1 1 ) / P d M (1 1 1 ) E e A Q 3 1 3 1 E P d (1 1 1 ) / P d
3M 1
(1 1 1 ) e A Q
, E P d (1 1 1 ) / P d
3M 1
(1 1 1 )
, and EeAQ are the total energy for the eAQ adsorbed
IP T
where
(7)
on Pd(1 1 1) or Pd3M1(1 1 1) surface, the energy of free Pd(1 1 1) or Pd3M1(1 1 1), and the
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energy of free eAQ, respectively. 3. Results and discussion
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3.1. XRD results
N
To confirm the structure of bimetallic monolithic catalysts after the addition of the
A
second metal (La or Zn), XRD characterization of these monolithic catalyst was conducted.
M
The results are shown in Fig. 1. For all of the monolithic catalysts, the characteristic peaks of
ED
COR correspond to the XRD standard PDF card no. 83-1385. The characteristic peaks at 2θ = 40.1 and 46.8° corresponding to Pd(111) and (200) crystal faces can be observed in the XRD
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pattern of Pd/SiO2/COR. However, these peaks for PdLa/SiO2/COR samples shift to smaller
CC E
2θ values and for PdZn/SiO2/COR samples shift to larger 2θ values, which can be attributed to the formation of Pd-M alloy. This is consistent with previous studies on analogous Pd-M bimetallic catalysts [31,32]. In addition, the lattice constant calculated by DFT for Pd-Zn
A
alloy Pd3Zn1 (3.92 Å) is smaller than pure Pd (3.94 Å), while for Pd3La1 (4.22 Å) is large than pure Pd (3.94 Å), which are consistent with the shifting of Pd(111) and Pd(200) peaks in XRD results. 3.2. H2-TPR results
12
The reducibilities of Pd/SiO2/COR, La/SiO2/COR, Zn/SiO2/COR, and Pd-M/SiO2/COR were investigated by H2-TPR. The results are shown in Fig. 2. There is a reduction peak for the La/SiO2/COR catalyst, which is attributed to the reduction of La2O3 species. For Pd/SiO2/COR monometallic monolithic catalyst, there are no reduction peaks detected.
IP T
However, there is a negative peak at 80 °C, which is attributed to the decomposition of PdH formed when the Pd species first interacts with hydrogen, indicating the Pd species can be
SC R
easily reduced when they contact with hydrogen even at low temperature [33,34]. The metal particle size is crucial to the thermal stability of PdH, as reported previously in literatures
U
[35-37]. Moreover, previous publications showed that the formed alloy in bimetallic catalysts
N
after the addition of the second metal (Ag, Cu and Bi) suppresses the formation of PdH
A
[38-40]. In this work, for the Pd-M/SiO2/COR bimetallic monolithic catalysts, there are not
M
peaks of PdH detected, confirming the formation of Pd-M alloy. It can be seen that the
ED
reduction temperatures decrease with the increase of Pd loading, indicating Pd in Pd-M/SiO2/COR bimetallic monolithic catalysts can promote the reduction of the second
PT
metal species. This may be attributed to the hydrogen spillover from Pd to the second metal
CC E
(M = La or Zn). It is noteworthy that the reduction temperature is remarkably influenced by the metal particle size. The reduction temperature of La/SiO2/COR is lower than that of Pd-La/SiO2/COR (M = L) bimetallic monolithic catalysts, which is attributed to the particle
A
size of La/SiO2/COR being smaller than that of Pd-La/SiO2/COR bimetallic monolithic catalysts. 3.3. SEM, TEM, and HRTEM results SEM was used for observing the microscopic change of monolithic catalysts, while
13
TEM was used for investigating the particle size distribution, providing evidence for the effect of the addition of the second metal on the surface metal distribution. The results are shown in Fig.3. SEM images of Pd-La/SiO2/COR monolithic catalyst show that the walls of Pd-La/SiO2/COR bimetallic monolithic catalyst are covered with a thin layer of SiO2
IP T
washcoat [41,42]. The results from TEM show that the metal particles are well dispersed. The mean particle sizes of 1.2% Pd/SiO2/COR, 1.2% La/SiO2/COR, 0.8% Pd-0.4% La/SiO2/COR,
SC R
0.6% Pd-0.6% La/SiO2/COR, and 0.4% Pd-0.8% La/SiO2/COR are 7.5, 2.9, 4.1, 3.5, and 3.1 nm, respectively, as calculated from 50 metal particles chosen from different regions. The
U
particle size decreases with the increase of La loading, which is consistent with the H2-TPR
N
results. The difference in surface energy between Pd (2.1 J m-2) and La (3.2 J m-2) results in
A
the different particle sizes of Pd/SiO2/COR and La/SiO2/COR monometallic monolithic
M
catalysts. That is, Pd diffuses faster than La, leading to the agglomeration of Pd [43]. Thus, a
ED
small amount addition of La to Pd/SiO2/COR can prevent the agglomeration of Pd. The higher selectivity and H2O2 yield obtained by 0.8% Pd-0.4% La/SiO2/COR and 0.6%
PT
Pd-0.6% La/SiO2/COR can be attributed to the addition of La leading to the better dispersion
CC E
of Pd over these monolithic catalysts. Although the particle size of 0.4% Pd-0.8% La/SiO2/COR is smaller than those of 0.8% Pd-0.4% La/SiO2/COR and 0.6% Pd-0.6% La/SiO2/COR, the selectivity and H2O2 yield of 0.4% Pd-0.8% La/SiO2/COR is lower than
A
those of 0.8% Pd-0.4% La/SiO2/COR and 0.6% Pd-0.6% La/SiO2/COR, which is due to the Pd active sites being covered by the large amount of La. The results from HRTEM (see Fig .4) show that the lattice plane spacing of 1.2% Pd/SiO2/COR, 0.8% Pd-0.4% La/SiO2/COR, and 0.8% Pd-0.4% Zn/SiO2/COR are 0.227, 0.232, and 0.224 nm, respectively, which is
14
attributed to the (111) plane of Pd or Pd-M. This is due to the incorporation of the second metal into Pd, indicating the formation of Pd-M alloy. 3.4. XPS results To investigate the electronic effect in Pd-M/SiO2/COR after the addition of the second
IP T
metal, the chemical states of Pd, La, and Zn were investigated by XPS analysis. The results are shown in Fig. 5. For Pd /SiO2/COR, there are two main peaks of Pd3d spectrum for Pd
SC R
3d5/2 (335.3 eV) and Pd 3d3/2 (340.5 eV) of Pd0, which is consistent with the literatures [39,44]. For Pd-M/SiO2/COR, there is a shift of 0.2-1.0 eV toward to higher binding energy,
U
which is assigned to the charge transfer from Pd to the second metal. For La/SiO2/COR, the
N
binding energies of 836.7 eV and 851.5 eV for La 3d5/2 and La 3d3/2 correspond to the
A
reported values for pure La [45]. For Pd-La/SiO2/COR, the binding energies of La 3d5/2 and
M
La 3d3/2 shift to lower values, which is consistent with the Pd XPS data, confirming the
ED
electronic donation property of Pd in Pd-La/SiO2/COR bimetallic monolithic catalysts. For Zn/SiO2/COR, the binding energies of 1021.8 eV and 1045.1 eV for Zn 2p3/2 and Zn 2p1/2
PT
correspond to the reported values for pure Zn [20]. For Pd-Zn/SiO2/COR, the binding
CC E
energies of Zn 2p3/2 and Zn 2p1/2 shift to lower values, which is consistent with the Zn XPS data, also confirming the electronic donation property of Pd in Pd-Zn/SiO2/COR bimetallic monolithic catalysts. The synergistic electronic effect, by which Pd donates electron density
A
to the second metal, can promote the products to remove from the surface of monolithic catalysts, thus increasing the catalytic activity and selectivity. 3.5. H2–O2 titration The Pd dispersion and Pd-specific surface area of Pd/SiO2/COR and Pd-M/SiO2/COR
15
were investigated by the H2–O2 titration [23], which are remarkably influenced by the second metal. The results are shown in Table 1. The Pd dispersion of Pd-La/SiO2/COR is higher than that of Pd-Zn/SiO2/COR, although the Pd loadings are identical. The results confirm that La is more conductive to improve the Pd dispersion of bimetallic monolithic catalysts and stop
IP T
the Pd metal particles from migration and aggregation, indicating the stronger interaction between Pd and La, consistent with the TEM, H2-TPR, and XPS characterizations as
SC R
mentioned above. Compared to Pd/SiO2/COR, the Pd dispersion and Pd-specific surface area
of Pd-M/SiO2/COR are higher. A small amount addition of La or Zn to Pd/SiO2/COR can
U
prevent the agglomeration of Pd, consistent with the TEM characterization, indicating the
N
strong synergistic effect between Pd and La or Zn.
A
3.6. Catalytic hydrogenation of eAQ
M
The catalytic performance of Pd/SiO2/COR monometallic and Pd-M/SiO2/COR
ED
bimetallic monolithic catalysts in the hydrogenation of eAQ was evaluated in the stainless steel fixed-bed reactor aforementioned. The results are shown in Table 1 and Fig. 6. The
PT
effect of the second metal (La or Zn) on the selectivity and H2O2 yield was investigated in the
CC E
hydrogenation of eAQ. It was found that Pd-La/SiO2/COR has higher selectivity and H2O2 yield than Pd-Zn/SiO2/COR, while Pd/SiO2/COR has higher selectivity and H2O2 yield than La/SiO2/COR. The selectivity and H2O2 yield of Pd-La/SiO2/COR are higher than those of
A
Pd/SiO2/COR and La/SiO2/COR. Moreover, the selectivity and H2O2 yield of the mixture of Pd/SiO2/COR and La/SiO2/COR are lower than those of Pd-La/SiO2/COR, confirming the strong synergy between Pd and La in the bimetallic monolithic catalyst. The better catalytic performance of Pd-La/SiO2/COR is not only attributed to the synergy between Pd and the
16
second metal, as proved by the results of TEM, XPS, and TPR characterizations, but also attributed to the strong interaction between Pd3La1 (1 1 1) and the carbonyl group (C=O) of eAQ, as proved by DFT calculations. Since the metal loading plays an important role in the selectivity and H2O2 yield of
IP T
monometallic and bimetallic monolithic catalysts, the catalytic performance of a series of monolithic catalysts with various mass ratios of Pd/M (M =La or Zn) was investigated. The
SC R
results are shown in Fig. 6. It is noteworthy that the Pd-M/SiO2/COR bimetallic monolithic catalysts with the mass ratio of Pd/M = 2 achieve the highest selectivity and H2O2 yield,
U
which is attributed to the better Pd dispersion after the addition of La or Zn. The selectivity
N
and H2O2 yield decrease with the increase of mass ratio of Pd/M at mass ratio of Pd/M above
A
2, which may be due to the agglomeration of Pd. The selectivity and H2O2 yield increase with
M
the increase of mass ratio of Pd/M at mass ratio of Pd/M below 2, which is attributed to the
ED
better Pd dispersion after the addition of La or Zn. In addition, 0.8% Pd-0.4% M/SiO2/COR bimetallic monolithic catalysts has higher TOF
PT
than 1.2% Pd/SiO2/COR monometallic monolithic catalyst, which is due to the addition of
CC E
the second metal leading to better Pd dispersion. 3.7. DFT calculations on Pd(1 1 1) and Pd3M1(1 1 1) surfaces It is noteworthy that the hydrogenation of AQ over Pd pellet catalyst follows the
A
Langmuir-Hinshelwood (L-H) mechanism, in which the overall activation barrier of the hydrogenation of AQ is low (only 16.1 kcal mol−1) and the activation barrier for the decomposition of H2 on the top site of a Pd atom is also low (only 3.9 kcal·mol-1) [46]. Thus, the adsorption of eAQ over Pd or Pd-M surfaces of monolithic catalysts is vital to the eAQ
17
hydrogenation. In this work, the adsorption energies of eAQ over both Pd and PdM surfaces were calculated by DFT to investigate the difference of catalytic performance over Pd monometallic and Pd-M bimetallic monolithic catalysts. Herein, the two Pd3-La1 and Pd3-Zn1
IP T
crystal structures, along with pure Pd, were applied for modeling the Pd-M alloy. The results are shown in Fig. 7.
SC R
However, there are numerous possible configurations for the adsorption of eAQ over
Pd(1 1 1) and Pd3M1(1 1 1) surfaces. Thus, according to previous publications [46], the most
U
relevant structures of the adsorption of eAQ over these surfaces were selected. The most
N
stable and possible adsorption structures are shown in Fig. 8 and Fig. 9, respectively. The
A
adsorption energies (Eads) and bond lengths are given in Tables 2 and 3.
M
The most stable adsorption structure of eAQ over the optimized Pd(1 1 1) surface is that
ED
carbonyl oxygen atoms of eAQ and two benzene rings are located at bridge sites, in which the carbonyl bond is elongated by 0.048 when compared to single eAQ. That may be
PT
assigned to the interaction between the carbonyl π bond and the Pd(1 1 1) surface (i.e.,
CC E
electron back-donation), as confirmed in literatures [47]. The benzene rings of eAQ play an crucial role in promoting a flat eAQ adsorption on Pd(1 1 1) surface, which is consistent with the furan adsorption on Pd(1 1 1) by DFT calculations [48], indicating that the preferred
A
adsorption structure is with the benzene ring essentially parallel to the metal surface. However, the benzene rings have a strong affinity for Pd instead of the second metals [47]. Thus, the adsorption energy of eAQ over Pd(1 1 1) surface is 61.4 kcal·mol-1. The adsorption energies of eAQ over Pd3La1(1 1 1) and Pd3Zn1(1 1 1) decrease to 57.9 and 55.7 kcal·mol-1,
18
respectively. However, the adsorption energies of eAQ on Pd3La1(1 1 1) and Pd3Zn1(1 1 1) don’t change so much when compared to that on Pd(1 1 1), due to the strong bonding between carbonyl group and the second metal atoms. Compared to the adsorption of eAQ on Pd (1 1 1), the bond lengths (C=O, Pd-C/M-C, and Pd-O) (M = La and Zn) are elongated,
IP T
showing the weaker binding benzene rings and the stronger binding carbonyl group (see Tables 2 and 3). At the same time, the bond lengths of M-O (M =La and Zn) are shortened
SC R
apparently when compared to Pd-O, indicating the strong bonding between carbonyl group and the second metal atoms. Thus, the stronger adsorption of carbonyl group with the Pd-M
U
alloy surface results in the higher catalytic activity when compared to Pd/SiO2/COR
N
monometallic monolithic catalyst.
A
4. Conclusions
M
In this work, a series of Pd-La/SiO2/COR and Pd-Zn/SiO2/COR bimetallic monolithic
ED
catalysts were prepared by the co-impregnation method. The 0.8% Pd-0.4% La/SiO2/COR bimetallic monolithic catalyst presents the highest H2O2 yield (7.7 g·L-1) and selectivity (95%)
PT
compared to 1.2% Pd /SiO2/COR monometallic monolithic catalyst (4.1 g·L-1 and 60%). This
CC E
is attributed to the smaller metal particle and the strong interaction between Pd and the second metal after the addition of La, as proved by the characterization results of TEM, H2-TPR, and XPS.
A
Moreover, the adsorption of eAQ over Pd (1 1 1), Pd3La1(1 1 1) and Pd3Zn1(1 1 1) was
investigated by DFT calculations. It was found that the stronger adsorption between the Pd-La alloy surface and the carbonyl group of eAQ results in the higher catalytic activity. This work provides some theoretical insights into the reaction mechanism for eAQ
19
A
CC E
PT
ED
M
A
N
U
SC R
IP T
hydrogenation on Pd monometallic and Pd-M bimetallic monolithic catalysts.
20
Acknowledgement This work is financially supported by the Fundamental Research Funds for the Central Universities (XK1802-1).
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IP T
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pre-reduction on the properties and the catalytic activity of Pd/carbon catalysts: A comparison with Pd/Al2O3, ACS Catal. 4 (2014) 187–194.
CC E
[34] E. Groppo, G. Agostini, A. Piovano, N.B. Muddada, G. Leofanti, R. Pellegrini, G. Portale, A. Longo, C. Lamberti, Effect of reduction in liquid phase on the properties and the catalytic activity of Pd/Al2O3 catalyst, J. Catal. 287 (2012) 44–54.
A
[35] S. Gómez-Quero, F. Cárdenas-Lizana, M.A. Keane, Effect of metal dispersion on the liquid-phase hydrodechlorination of 2,4-Dichlorophenol over Pd/Al2O3, Ind. Eng. Chem. Res. 47 (2008) 6841–6853. [36] H.P. Aytam, V. Akula, K. Janmanchi, S.R.R. Kamaraju, K.R. Panja, Characterization and reactivity of Pd/MgO and Pd/γ-Al2O3 catalysts in the selective hydrogenolysis of CCl2F2, J. Phys. Chem. B 106 (2002) 1024–1031. 23
[37] Y. Liu, Y. He, D. Zhou, J. Feng, D. Li, Catalytic performance of Pd promoted Cu hydrotalcite-derived catalysts in partial hydrogenation of acetylene: effect of Pd–Cu alloy formation, Catal Sci Technol. 6 (2016) 3027–3037. [38] S. Karski, I. Witońska, J. Rogowski, J. Gołuchowska, Interaction between Pd and Ag on the surface of silica, J. Mol. Catal. A: Chem. 240 (2005) 155–163.
Catal. A: Chem. 191 (2003) 87–92.
IP T
[39] S. Karski, I. Witońska, Bismuth as an additive modifying the selectivity of palladium catalysts, J. Mol.
[40] Q. Zhang, J. Li, X. Liu, Q. Zhu, Synergetic effect of Pd and Ag dispersed on Al2O3 in the selective
SC R
hydrogenation of acetylene, Appl. Catal. A: Gen. 197 (2000) 221–228.
[41] O. Sanz, F.J. Echave, M. Sánchez, A. Monzon, M. Montes, Aluminium foams as structured supports for volatile organic compounds (VOCs) oxidation, Appl. Catal. A: Gen. 340 (2008) 125–132.
U
[42] B. Yuan, B. Zhang, Z. Wang, S. Liu, J. Li, Y. Liu, C. Li, Photocatalytic aerobic oxidation of toluene
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and its derivatives to aldehydes on Pd/Bi2WO6, Chin. J. Catal. 38 (2017) 440–446.
A
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M
[44] Q. Zhang, J. Li, X Liu, Q. Zhu. Synergetic effect of Pd and Ag dispersed on Al2O3 in the selective
ED
hydrogenation of acetylene, Appl. Catal. A: Gen. 197 (2000) 221–228. [45] Q. Wang, G. Li, B. Zhao, M. Shen, R. Zhou, The effect of La doping on the structure of Ce0.2Zr0.8O2 and the catalytic performance of its supported Pd-only three-way catalyst, Appl. Catal. B: Environ.
PT
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CC E
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24
Table Captions
Table 1. Catalytic performance and textural parameters of different monolithic catalysts for the
IP T
hydrogenation of eAQ to eAQH2.
Table 2. Adsorption energies and optimized bond lengths of the most stable DFT optimized eAQ on Pd(1 1
SC R
1) and on Pd3M1(1 1 1) (M = La, and Zn) slabs.
U
Table 3. Adsorption energies and optimized bond lengths of the optimized possible structures of eAQ on
A
CC E
PT
ED
M
A
N
Pd(1 1 1) and on Pd3M1(1 1 1) (M = La, and Zn) slabs.
25
I N U SC R
Table 1
Catalytic performance and textural parameters of different monolithic catalysts for the hydrogenation of eAQ to eAQH2.
Pd-loading (wt.%)
Second metal-loading (wt.%)
Selectivity (%)
Hydrogenation efficiency (g·L-1)
-
60
4.1
805
17
86
0.72
20
0.5
-
-
-
0.2
A
Washcoat
DPd (%)a
SPd (m2·g-1)
TOF （s-1）
30
La/SiO2/COR
ED
M
Sample
Washcoat loading (wt.%)
Pd efficiency (g H2O2·g-1 Pd·h-1)
SiO2
30
1.2
Pd-La/SiO2/COR
SiO2
30
0.8
0.4
95
7.7
1385
38
196
1.31
Pd-Zn/SiO2/COR
SiO2
30
0.8
0.4
86
6.5
950
31
157
1.1
SiO2
30
0.8
0.4
78
4.5
900
-
-
0.88
SiO2
1.2
A
CC E
PT
Pd/SiO2/COR
Pd/SiO2/COR+La/SiO2/COR
DPda was determined by H2-O2 titration analysis.
26
Table 2 Adsorption energies and optimized bond lengths of the most stable DFT optimized eAQ on Pd(1 1 1) and on Pd3M1(1 1 1) (M = Ni, Fe, Mn, and Cu) slabs.
C-O (Å)
Pd-Ca (Å)
Pd-Ob (Å)
M-Cc (Å)
M-Od (Å)
eAQ
-
1.237
-
-
-
-
eAQ/Pd(1 1 1)
61.4
1.285/1.286
2.276/2.268
2.382/2.372
-
-
eAQ/Pd3La1(1 1 1)
57.9
1.327/1.331
2.778/2.875
2.787
-
2.476
eAQ/Pd3Zn1(1 1 1)
55.7
1.297/1.275
2.349/2.397
-
2.699
SC R
U
N
2.387
A
CC E
PT
ED
M
A
Pd-Ca: the distance between C and the nearest Pd atom, as shown in Fig. 8. Pd-Ob: the distance between O and the nearest Pd atom, as shown in Fig. 8. M-Cc: the distance between C and the nearest second metal atom, as shown in Fig. 8. M-Od: the distance between O and the nearest second metal atom, as shown in Fig. 8.
27
IP T
Eads (kcal·mol-1)
model
Table 3 Adsorption energies and optimized bond lengths of the optimized possible structures of eAQ on Pd(1 1 1)
Pd-Ca (Å)
Pd-Ob (Å)
eAQ
-
1.237
-
-
eAQ/Pd(1 1 1)
61.4
1.285/1.286
2.276/2.268
eAQ/Pd3La1(1 1 1)-1
55.8
1.333/1.274
2.475/2.570
eAQ/Pd3La1(1 1 1)-2
53.5
1.330/1.333
3.141
eAQ/Pd3La1(1 1 1)-3
49.4
1.330/1.334
eAQ/Pd3Zn1(1 1 1)-1
53.9
1.299/1.276
eAQ/Pd3Zn1(1 1 1)-2
51.8
1.285/1.284
1.286/1.285
PT
CC E A
-
2.945
-
2.310
3.080/3.437
2.436
-
3.014/3.194
2.522
2.056
2.009/2.334
2.379
-
2.385/2.465
2.577
-
2.477
2.542
2.475/2.698
2.678
-
U
-
3.141
Pd-Ca: the distance between C and the nearest Pd atom, as shown in Fig. 9. Pd-Ob: the distance between O and the nearest Pd atom, as shown in Fig. 9. M-Cc: the distance between C and the nearest second metal atom, as shown in Fig. 9. M-Od: the distance between O and the nearest second metal atom, as shown in Fig. 9.
28
-
N
47.3
M-Od (Å)
2.382/2.372
A
ED
eAQ/Pd3Zn1(1 1 1)-3
M-Cc (Å)
IP T
C-O (Å)
model
SC R
Eads (kcal·mol-1)
M
and on Pd3M1(1 1 1) (M = Ni, Fe, Mn, and Cu) slabs.
Figure Captions Fig. 1. XRD patterns of the monolithic catalysts. (a), 1.2% Pd/SiO2/COR; (b), 1.2% La/SiO2/COR; (c), 1.2% Zn/SiO2/COR; (d), 0.8% Pd-0.4% La/SiO2/COR; (e), 0.6% Pd-0.6% La/SiO2/COR; (f), 0.4% Pd-0.8% La/SiO2/COR; (g), 0.8% Pd-0.4%
Fig. 2. H2-TPR spectra of different monolithic catalysts.
SC R
IP T
Zn/SiO2/COR; (h), 0.6% Pd-0.6% Zn/SiO2/COR; (i), 0.4% Pd-0.8% Zn/SiO2/COR.
(A) Pd-La/SiO2/COR. (a) 1.2% Pd/SiO2/COR; (b) 1.2% La/SiO2/COR; (c) 0.8% Pd-0.4% La /SiO2/COR;
U
(d) 0.6% Pd-0.6% La /SiO2/COR; (e) 0.4% Pd-0.8% La /SiO2/COR.
N
(B) Pd-Zn/SiO2/COR. (a) 1.2% Pd/SiO2/COR; (b) 1.2% Zn/SiO2/COR; (c) 0.8% Pd-0.4% Zn/SiO2/COR;
M
A
(d) 0.6% Pd-0.6% Zn/SiO2/COR; (e) 0.4% Pd-0.8% Zn/SiO2/COR.
ED
Fig. 3. SEM images of 0.8% Pd-0.4% La/SiO2/COR catalyst (a, b); TEM images of 1.2% Pd/SiO2/COR (c, d); 1.2% La/SiO2/COR (e, f); 0.8% Pd-0.4% La/SiO2/COR (g, h); 0.6% Pd-0.6% La/SiO2/COR (i, j); and
PT
0.4% Pd-0.8% La /SiO2/COR (k, l). Pd particle size distributions of 1.2% Pd/SiO2/COR (m); 1.2%
CC E
La/SiO2/COR (n); 0.8% Pd-0.4% La/SiO2/COR (o); 0.6% Pd-0.6% La/SiO2/COR (p); and 0.4% Pd-0.8% La /SiO2/COR (q).
A
Fig. 4. HRTEM images of 1.2% Pd/SiO2/COR (a); 0.8% Pd-0.4% La/SiO2/COR (b); 0.8% Pd-0.4% Zn/SiO2/COR (c).
Fig. 5. Pd3d, La3d, and Zn2p spectra of different monolithic catalysts.
29
(A) Pd3d (a) 1.2% Pd/SiO2/COR; (b) 0.8% Pd-0.4% La/SiO2/COR; (c) 0.6% Pd-0.6% La/SiO2/COR; (d) 0.4% Pd-0.8% La/SiO2/COR. (B) La3d (a) 1.2% La/SiO2/COR; (b) 0.8% Pd-0.4% La/SiO2/COR; (c) 0.6% Pd-0.6% La/SiO2/COR; (d) 0.4% Pd-0.8% La/SiO2/COR.
IP T
(C) Pd3d (a) 1.2% Pd/SiO2/COR; (b) 0.8% Pd-0.4% Zn/SiO2/COR; (c) 0.6% Pd-0.6% Zn/SiO2/COR; (d) 0.4% Pd-0.8% Zn/SiO2/COR.
SC R
(D) Zn2p (a) 1.2% Zn/SiO2/COR; (b) 0.8% Pd-0.4% Zn/SiO2/COR; (c) 0.6% Pd-0.6% Zn/SiO2/COR; (d)
U
0.4% Pd-0.8% Zn/SiO2/COR.
N
Fig. 6. Effect of the second metal loadings on selectivity (a) and H2O2 yield (b).
A
Reaction conditions: pressure = 101 kPa, temperature = 80°C, concentration of eAQ solution = 60 g·L-1,
M
flow rate of eAQ solution = 10 mL·min-1, and flow rate of H2 gas = 100 mL·min-1.
ED
A, 1.2% Pd/SiO2/COR; B, 0.96% Pd-0.24% M/SiO2/COR; C, 0.8% Pd-0.4% M/SiO2/COR; D, 0.6%
PT
Pd-0.6% M/SiO2/COR; E, 0.4% Pd-0.8%. M/SiO2/COR; F, 0.24% Pd-0.96% M/SiO2/COR.
CC E
Fig. 7. Bulk monometallic Pd and bimetallic Pd-M (M = La and Zn) alloy structures.
Fig. 8. The most stable DFT optimized structures of eAQ (a) and eAQ adsorbed on Pd(1 1 1) (b), Pd3La1(1
A
1 1) (c), and Pd3Zn1(1 1 1) (d).
Fig. 9. The possible structures optimized by DFT of eAQ (a) and eAQ adsorbed on Pd3La1(1 1 1)-1 (b), Pd3La1(1 1 1)-2 (c), Pd3La1(1 1 1)-3 (d), Pd3Zn1(1 1 1)-1 (e), Pd3Zn1(1 1 1)-2 (f), and Pd3Zn1(1 1 1)-3 (g).
30
(i)
● cordierite
●
● ●●
●● ●
●
(i)
intensity
(h) (g) (f) (e)
IP T
(d) (c) (b)
10
20
30
40
2θ (degree)
● cordierite ▼ Pd ★ PdLa
★ ▼
42
44
46
U
◆
◆
◆
●
▼
(a)
48
40
50
2θ (degree)
42
44
46
(i)
◆
(h)
◆
(g)
▼
(a)
(d)
ED
▼
(200)
●
A
(e)
★
★
◆
(f)
intensity
●
★
★
40
(111)
M
intensity
●
●
★
● cordierite ▼ Pd ◆ PdZn
(iii)
(200)
(111)
50
N
(ii)
SC R
(a)
48
50
CC E
PT
2θ (degree)
Fig. 1. XRD patterns of the monolithic catalysts. (a), 1.2% Pd/SiO2/COR; (b), 1.2% La/SiO2/COR; (c), 1.2% Zn/SiO2/COR; (d), 0.8% Pd-0.4%
A
La/SiO2/COR; (e), 0.6% Pd-0.6% La/SiO2/COR; (f), 0.4% Pd-0.8% La/SiO2/COR; (g), 0.8% Pd-0.4% Zn/SiO2/COR; (h), 0.6% Pd-0.6% Zn/SiO2/COR; (i), 0.4% Pd-0.8% Zn/SiO2/COR.
31
476
(A)
(e) 442
(d)
387
Intensity
(c) 355
(a)
400
600
T (C)
576
(B)
800
SC R
200
50
IP T
(b)
U
(e)
N
443
200
400
(d) (c) (b) (a)
600
800
T (C)
CC E
PT
50
ED
M
324
A
Intensity
381
Fig. 2. H2-TPR spectra of different monolithic catalysts. (A) Pd-La/SiO2/COR. (a) 1.2% Pd/SiO2/COR; (b) 1.2% La/SiO2/COR; (c) 0.8% Pd-0.4% La /SiO2/COR;
A
(d) 0.6% Pd-0.6% La /SiO2/COR; (e) 0.4% Pd-0.8% La /SiO2/COR. (B) Pd-Zn/SiO2/COR. (a) 1.2% Pd/SiO2/COR; (b) 1.2% Zn/SiO2/COR; (c) 0.8% Pd-0.4% Zn/SiO2/COR; (d) 0.6% Pd-0.6% Zn/SiO2/COR; (e) 0.4% Pd-0.8% Zn/SiO2/COR.
32
IP T
(b)
M
A
N
U
SC R
(a)
50 nm
20 nm (d)
A
CC E
PT
ED
(c)
10 nm
50 nm (e)
(f)
33
IP T
50 nm
20 nm (h)
M
A
N
U
SC R
(g)
10 nm
ED
50 nm (j)
A
CC E
PT
(i)
50 nm
10 nm
(k)
(l)
34
70
35
dm = 2.9 nm
30
60
25
50
counts (%)
20 15
40 30
10
20
5
10 0
0 2
4
6
8
10
12
2
14
4
8
10
SC R
45
70
dm = 3.5nm
40
dm = 4.1 nm
60
6
Diameter (nm) (n)
Diameter (nm) (m)
30
25 20 15
20
10
A
5
10
U
40
30
N
counts (%)
35
50
counts (%)
IP T
counts (%)
dm = 7.5 nm
0
0 4
6
8
Diameter (nm) (o)
2
4
6
8
10
Diameter (nm) (p)
dm =3.1 nm
ED
60
10
M
2
55 45 40 35
PT
counts (%)
50
30 25
A
CC E
20 15 10
5 0 2
4
6
8
10
Diameter (nm) (q)
Fig. 3. SEM images of 0.8% Pd-0.4% La/SiO2/COR catalyst (a), (b); TEM images of 1.2% Pd/SiO2/COR (c), (d); 1.2% La/SiO2/COR (e), (f); 0.8% Pd-0.4% La/SiO2/COR (g), (h); 0.6% Pd-0.6% La/SiO2/COR (i), (j); and 0.4% Pd-0.8% La /SiO2/COR (k), (l). Pd particle size distributions of 1.2% Pd/SiO2/COR (m); 1.2% La/SiO2/COR (n); 0.8% Pd-0.4% La/SiO2/COR (o); 0.6% Pd-0.6% La/SiO2/COR (p); and 0.4% Pd-0.8% La /SiO2/COR (q). 35
d = 0.227 nm
d=0.235 nm
IP T
d=0.224 nm
5 nm
5 nm (b)
(c)
SC R
(a)
Fig. 4 HRTEM images of 1.2% Pd/SiO2/COR (a); 0.8% Pd-0.4% La/SiO2/COR (b); 0.8% Pd-0.4%
A
CC E
PT
ED
M
A
N
U
Zn/SiO2/COR (c).
36
Pd3d3/2
Pd3d5/2
(B)
(A) 340.5
851.5
836.7
(a)
(a) (b)
Intensity
Intensity
335.3
La3d3/2
La3d5/2
(b)
(c)
(c)
330
332
334
336
338
340
342
344
830
835
840
Binding energy (eV)
Zn2p3/2
Pd3d3/2
(C)
(D) 340.5
336
338
340
342
U
(a) (b) (c)
344
1015
1020
(d) 1025
1030
1035
1040
1045
1050
Binding energy (eV)
ED
Binding energy (eV)
M
(d)
334
1045.1
N
(c)
332
855
Zn2p1/2
A
Intensity
(b)
330
1021.8
(a)
Intensity
335.3
850
Binding energy (eV)
SC R
Pd3d5/2
845
IP T
(d) (d)
PT
Fig. 5. Pd3d, La3d, and Zn2p spectra of different monolithic catalysts. (A) Pd3d (a) 1.2% Pd/SiO2/COR; (b) 0.8% Pd-0.4% La/SiO2/COR; (c) 0.6% Pd-0.6% La/SiO2/COR; (d)
CC E
0.4% Pd-0.8% La/SiO2/COR.
(B) La3d (a) 1.2% La/SiO2/COR; (b) 0.8% Pd-0.4% La/SiO2/COR; (c) 0.6% Pd-0.6% La/SiO2/COR; (d) 0.4% Pd-0.8% La/SiO2/COR.
A
(C) Pd3d (a) 1.2% Pd/SiO2/COR; (b) 0.8% Pd-0.4% Zn/SiO2/COR; (c) 0.6% Pd-0.6% Zn/SiO2/COR; (d) 0.4% Pd-0.8% Zn/SiO2/COR. (D) Zn2p (a) 1.2% Zn/SiO2/COR; (b) 0.8% Pd-0.4% Zn/SiO2/COR; (c) 0.6% Pd-0.6% Zn/SiO2/COR; (d) 0.4% Pd-0.8% Zn/SiO2/COR.
37
100
60
40
IP T
Selectivity (%)
80
PdLa PdZn Pd
0
A
B
C
D
E
7
PdLa PdZn Pd
N
8
A M
5 4 3
1
PT
0
ED
H2O2 yield (g·L-1)
6
2
F
U
Catalysts
SC R
20
B
C
D
E
F
Catalysts
CC E
A
Fig. 6. Effect of the second metal loadings on selectivity (a) and H2O2 yield (b). Reaction conditions: pressure = 101 kPa, temperature = 80°C, concentration of eAQ solution = 60 g·L-1,
A
flow rate of eAQ solution = 10 mL·min-1, and flow rate of H2 gas = 100 mL·min-1. A, 1.2% Pd/SiO2/COR; B, 0.96% Pd-0.24% M/SiO2/COR; C, 0.8% Pd-0.4% M/SiO2/COR; D, 0.6% Pd-0.6% M/SiO2/COR; E, 0.4% Pd-0.8%. M/SiO2/COR; F, 0.24% Pd-0.96% M/SiO2/COR.
38
SC R
IP T
Pd
Pd3Zn1
ED
M
A
N
U
Pd3La1
CC E
PT
Pd
La
A
Fig. 7. Bulk monometallic Pd and bimetallic Pd-M (M = La and Zn) alloy structures.
39
Zn
(a)
IP T
1.237
SC R
(b) 2.382
1.285
U
2.276
2.268
2.372
A
N
1.286
ED
M
Eads = 61.4 kcal·mol-1
2.787
PT
(c)
1.327
A
CC E
2.778 1.331
2.875 2.476
Eads = 57.9 kcal·mol-1
40
(d) 2.387 1.297 2.349 2.397 1.275
IP T
2.699
O
H
Pd
La
Zn
M
A
N
U
C
SC R
Eads = 55.7 kcal·mol-1
ED
Fig. 8. The most stable DFT optimized structures of eAQ (a) and eAQ adsorbed on Pd(1 1 1) (b), Pd3La1(1
A
CC E
PT
1 1) (c), and Pd3Zn1(1 1 1) (d).
41
(a)
SC R
IP T
1.237
(b)
1.333
2.475 1.274
A
N
U
2.310
M
2.945
Eads = 55.8 kcal·mol-1
CC E
(c)
PT
ED
Pd3La1(1 1 1)-1
2.570
3.080 1.330 3.141
A
2.436 1.333 3.437
Pd3La1(1 1 1)-2
Eads = 53.5 kcal·mol-1
42
(d)
3.014 1.330 2.522
Pd3La1(1 1 1)-3
Eads = 49.4 kcal·mol-1
2.009
1.299 2.056 2.379 1.276 2.334
ED
M
A
N
U
(e)
SC R
3.194
CC E
(f)
Eads = 53.9 kcal·mol-1
PT
Pd3Zn1(1 1 1)-1
2.447 1.285 2.385
A
2.465 1.284 2.577
Pd3Zn1(1 1 1)-2
Eads = 51.8 kcal·mol-1
43
IP T
3.141 1.334
(g) 2.475
1.286
2.678 2.542 2.698
Pd3Zn1(1 1 1)-3
O
H
Pd
La
Zn
M
A
N
C
U
SC R
Eads = 47.3 kcal·mol-1
IP T
1.285
ED
Fig. 9. The possible structures optimized by DFT of eAQ (a) and eAQ adsorbed on Pd3La1(1 1 1)-1 (b),
A
CC E
PT
Pd3La1(1 1 1)-2 (c), Pd3La1(1 1 1)-3 (d), Pd3Zn1(1 1 1)-1 (e), Pd3Zn1(1 1 1)-2 (f), and Pd3Zn1(1 1 1)-3 (g).
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