Mechanochemical Nonhydrolytic Sol-Gel-Strategy for the Production of Mesoporous Multimetallic Oxides

Article abstract of DOI:10.1021/acs.chemmater.9b01244

Mesoporous metal oxides with wide pore size, high surface area, and uniform porous structures have demonstrated excellent advantages in various fields. However, the state-of-art synthesis approaches are dominated by wet chemistry, accompanied by use of ex

Full text of DOI:10.1021/acs.chemmater.9b01244

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Article
A Mechanochemical Non-hydrolytic Sol-Gel-Strategy
for the Production of Mesoporous Multi-metallic Oxides
Zihao Zhang, Shize Yang, Xiaobing Hu, Haidi Xu, Honggen Peng, Miaomiao
Liu, Bishnu Prasad Thapaliya, Kecheng Jie, Jiahua Zhao, Jixing Liu, Hao
Chen, Yan Leng, Xiuyang Lu, Jie Fu, Pengfei Zhang, and Sheng Dai
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01244 • Publication Date (Web): 01 Jul 2019
Downloaded from http://pubs.acs.org on July 2, 2019
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A Mechanochemical Non-hydrolytic Sol-Gel-Strategy for the
Production of Mesoporous Multi-metallic Oxides
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Zihao Zhang,abc Shize Yang, Xiaobing Hu, Haidi Xu, Honggen Peng, Miaomiao Liu,
e
fg
c
b
c
c
c
d
c
a
c
Bishnu Prasad Thapaliya, Kecheng Jie, Jiahua Zhao, Jixing Liu, Hao Chen, Yan Leng,
a
a
d
bc
Xiuyang Lu, Jie Fu, * Pengfei Zhang * and Sheng Dai *
a
Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of
Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China
b
Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA
c
Department of Chemistry, The University of Tennessee, Knoxville, TN, USA
d
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai
2
00240, China
e
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY11973,
USA
f
Department of Materials Science and Engineering, Northwestern University, Evanston, IL
6
0208, USA
g
The NUANCE Center, Northwestern University, Evanston, IL 60208, USA
*
Corresponding author: Jie Fu, E-mail: jiefu@zju.edu.cn
Pengfei Zhang, E-mail: chemistryzpf@sjtu.edu.cn
Sheng Dai, E-mail: dais@ornl.gov
Abstract: Mesoporous metal oxides with wide pore size, high surface area and uniform
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porous structures have demonstrated excellent advantages in various fields. However, the
state-of-art synthesis approaches are dominated by wet chemistry, accompanied by use of
excessive solvent, and the requirement of time-consuming drying process. Herein, we report
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a mechanochemical solid-state route to synthesize mesoporous Al
2
O
3
(meso-Al
2
3
O ) via
aluminum isopropoxide-copolymers assembly. The obtained meso-Al
2
O
3
shows a record
2
-1
high surface area (~644 m g ) and narrow pore size distribution (centered at ~5 nm).
Moreover, a mechanochemical nonhydrolytic sol-gel strategy is introduced to fabricate
mesoporous transition metal (Cu, Co, Mn, Fe, Mg, Ni)-aluminium binary oxide by using
anhydrous metal chlorides and aluminum isopropoxide interplay. More importantly, four or
five metals-aluminum oxide complexes with abundant mesopores and single cubic crystalline
phase known as high-entropy ceramics are produced. To the best of our knowledge,
mesoporous high-entropy metal oxides have not been prepared before, because the high
crystallization temperature would make mesopores collapse. Additionally, this high-entropy
property endows (CuNiFeCoMg)O
x
-Al
2
O
3
with superior SO -resisting performance (1000
2
ppm SO in N at 280 °C) in the catalytic oxidation of CO compared to single CuO-Al
2
2
2
3
O .
1
. Introduction
Mesoporous metal oxides with wide pore size, high surface area, uniform pore size
distribution as well as a variety of structures and compositions endow them particularly
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attractive for applications in catalysis, adsorption, battery technology, sensing, and so on.[1-
6
]
Template-assisted processes via either soft- or hard templates were usually used for the
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synthesis of mesoporous metal oxides.[7-12] In general, the hard-templating technique relied
on mesoporous silica or carbon aerogel as the hard templates, which inevitably required
additional steps and costs.[9,13-21] In comparison, organic-inorganic assembly routes, using
block co-polymers or surfactants as soft templates have been developed as a more
straightforward method.[22-26] It is undeniable that the solvent evaporation-induced self-
assembly method has been successfully applied into the synthesis of many mesoporous metal
oxides.[27-29] However, several issues seem left in this wet soft-templating process, including:
1
) Excessive organic solvents are used; 2) It requires the metal precursors that have to be
dissolved in the solvents, which limits the applications of many insoluble metal
precursors;[3,30] 3) The slow solvent evaporation is a time-consuming step.[31]
To resolve these disadvantages, solid-state synthesis, an old method in material
processing, has been revisited.[32-35] Several porous materials, such as zeolite, metal-organic
frameworks, covalent-organic frameworks, ordered mesoporous ploymers and mesoporous
carbons have been already synthesized via mechanochemical processes.[36-38] Although, a
solvent-free assembly method has also been introduced for the construction of mesoporous
metal oxide (TiO ), this route needs HCl as the solvent and aged at 140 °C for 24 h before
2
calcination, making it not a “true” solvent-free system.[37] As a result, the synthesis of
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mesoporous metal oxides via mechanochemical solid-state method, especially for metal
oxdies hybrids, is highly welcome. Moreover, alumina has been widely used as an important
carrier in catalysis, and alumina-based mixed mesoporous metal oxides also display
extensive application prospect.[39-43] Therefore, the one-pot preparation of mesoporous
alumina-supported metal oxides via mechanochemical solid-state method seems like a more
efficient and straightforward pathway toward industrial catalysis.
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Herein, we demonstrate the facile, rapid and solid-state synthesis of mesoporous
aluminum oxide (meso-Al
2
3
O ) by a mechanochemical assembly between aluminum
isopropoxide and commercial polymers (e.g., PEO-PPO-PEO, P123; polyethylene glycol,
PEO, PEG). Moreover, binary Al-based oxides (Cu, Co, Mn et al.) with abundant mesopores
can be easily constructed by mechanochemical nonhydrolytic sol-gel process (NHSG, metal
chlorides and aluminum isopropoxide interplay).[44-46] More interestingly, four or five
transition metal species-aluminum oxide hybrids⎯the so-called high-entropy oxides⎯with
single cubic crystalline phase and rich mesoporous structure were synthesized via this
mechanochemical NHSG method. To our surprise, the high-entropy feature endows
(CuNiFeCoMg)O
x
-Al
2
O
3
with superior SO
2
-resisting performance (1000 ppm SO
2
in N at
2
2
80 °C) in the catalytic oxidation of CO compared to CuO-Al
2
3
O .
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2
. Experimental Section
2
.1 Catalyst synthesis
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meso-Al
2
3
O : 1 g metal precursor (aluminum isopropoxide) and 0.2 g ~ 0.4 g soft
-
1
-1
templates (Pluronic P123: 5800g mol ; PEG: 4000 g mol , Pluronic F127 (F127) and
cetyltrimethylammonium bromide (CTAB)) were added in a 9.5 mL screw-capped iron
reactor with two big (diameter: 1.2 cm) and three small (diameter: 0.5 cm) iron steel ball
bearings. The sealed reactor was placed in a high-speed vibrating ball miller (Retsch MM400),
and the mixtures were ball milled for 60 min at a vibrational frequency of 30 Hz. The
resulting samples were washed using ethanol and dried at 40 °C under vacuum conditions,
followed by calcinating at 400 °C with the heating rate of 2 °C min-1.
mesoporous binary metal oxides (Cu, Co, and Mn et al.): Aluminum isopropoxide (2.67
mmol) and a transition metal chloride (4 mmol) as well as 0.4 g PEG were added in in a 9.5
mL screw-capped iron reactor with two big (diameter: 1.2 cm) and three small (diameter: 0.5
cm) iron steel ball bearings. After ball milling for 60 min, the post-treatment procedure is
same with the synthesis of meso-Al
before use.
2
O . All catalysts were washed using water for three times
3
(
CuNiFeCo)
x
-Al
2
3
O : Aluminum isopropoxide (2.67 mmol), cooper chloride (1 mmol),
nickel chloride (1 mmol), cobalt chloride (1 mmol) and ferrous chloride (1 mmol), as well as
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.4 g PEG are added in in a 9.5 mL screw-capped iron reactor with two big (diameter: 1.2
cm) and three small (diameter: 0.5 cm) iron steel ball bearings. After ball milling for 60 min,
the post-treatment procedure is same with the synthesis of meso-Al
CuNiFeCoMg) -Al : Aluminum isopropoxide (3.33 mmol), cooper chloride (1
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O .
(
x
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mmol), nickel chloride (1 mmol), cobalt chloride (1 mmol), ferrous chloride (1 mmol) and
Magnesium chloride (1 mmol), as well as 0.4 g PEG are added in in a 9.5 mL screw-capped
iron reactor with two big (diameter: 1.2 cm) and three small (diameter: 0.5 cm) iron steel ball
bearings. After ball milling for 60 min, the post-treatment procedure is same with the
synthesis of meso-Al
2
3
O .
2
.2 Characterizations
X-rays diffraction (XRD) was collected on a PANalytical Empyrean diffractometer
operated at 45 kV and 40 mA (scanning step: 0.02° per step). N adsorption-desorption
2
isotherms were analyzed on a TriStar 3000 volumetric adsorption analyzer manufactured and
by Micromeritics Instrument Corp. A classical Barrett–Joyner–Halenda (BJH) method was
employed to determine the pore size distribution by using adsorption branch of isotherms.
Prior to analysis, all samples were activated at 160 °C for 8 h. Scanning electron microscopy
(SEM) with energy-dispersive X-ray spectroscopy (EDS) mapping results was recorded
using a Zeiss EVO-MA15 scanning electron microscope with a Bruker XFlash 6130 detector.
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Scanning electron microscopy (SEM) images was recorded using a Zeiss AURIGA.
Scanning transmission electron microscopy (STEM) was performed on a Nion microscope
operated at 200 kV and ARM 200CF equipped with a probe corrector. The images are
recorded using high angle annular dark field (HAADF) and bright field (BF) imaging. X-ray
photoelectron spectroscopy (XPS) was performed by AXIS Ultra DLD spectrometer (Kratos,
Japan). The obtained data was calibrated by C 1s standard peak and analyzed by Casa XPS
software. Temperature programmed desorption (TPD) of CO was recorded using FineSorb-
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010 from Zhejiang FINETEC INSTRUMENTS co., LTD with a thermal conductivity
detector (TCD). Prior to characterization, fresh and spent catalysts were preheated at 120 °C
under argon atmosphere. Elemental analysis of the samples was done by inductively coupled-
plasma atomic emission spectroscopy (ICP-AES) using the Optima 2100 DV spectrometer
(PerkinElmer Corporation). The actual metal ratios are summarized in Table S1. Fourier
Transform infrared spectroscopy (FTIR) spectra was collected on a Bio-Rad FTIR
spectrometer (Excalibur series).
2
.3 Experimental procedures
CO oxidation experiments were carried out in a fixed bed reactor (straight quartz tube
with 4 mm inner diameter) at atmospheric pressure. For the measurement of CO light-off
curves showing CO conversion as a function of reaction temperature, 20 mg of catalyst
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supported by quartz wool was loaded in the reactor. The feed gas of 1% CO balanced with
dry air (< 4 ppm water) passed though the catalyst bed at a flow rate of 10 mL/min
corresponding to a gas hourly space velocity (GHSV) of 30,000 mL/(h g cat). The
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concentrations of CO and CO in the reactor were analyzed by a Buck Scientific 910 gas
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chromatograph equipped with a dual molecular sieve/porous polymer column (Alltech CTR1)
and a thermal conductivity detector. CO conversion is calculated as the peak areas of CO of
consumption divided by that of CO before the reaction.
3
. Results and discussion
Scheme 1 summarizes a mechanochemical route for the synthesis of meso-Al
2
3
O and
related metal oxide hybrids using aluminum isopropoxide and anhydrous metal chlorides as
the precursors, and P123 or PEG as the soft template. An efficient assembly between
aluminum isopropoxide and P123 micelles is realized by mechanically grinding the mixture
in a vibrating ball miller for 60 min (much shorter than the traditional solvent evaporation
induced assembly process, 1 ~ 3 days in most cases).[23,47] It has been reported that the
condensation between chloride and alkoxide by mixing the chloride precursors (MCl
x
) and
oxygen donor (Al(OR)
3
) to produce the xerogels is considered as NHSG process[48]. The
resulting xerogels is then washed and dried under vacuum and finally calcined in air to
achieve the corresponding metal oxides. In this work, the mechanochemical NHSG process
between metal chlorides and aluminum isopropoxide in the solid-state via complex
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mechanism is restricted to the periphery of micelles. The obtained xerogels is performed by
XRD (Figure S1) and then calcinated at 400 °C in air to remove template. By combining with
aluminum isopropoxide-copolymers crosslinking and NHSG process in one-step, the mixed
oxides with uniform mesopores can be prepared.
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Scheme 1. The schematic diagram for the synthesis of meso-Al
2
3
O , mesoporous mixed metal
oxides using mechanochemical NHSG method.
The textural properties of the as-synthesized Al
surfactants are summaried (Table 1) by N adsorption-desorption isotherms at 77 K in Figure
S2. The blank Al sample without templates offers a low surface area (246 m g ) with a
broad pore size distribution in Figure S2a,c. In comparison, a meso-Al sample with 0.2g
P123 as the template gives a type-IV isotherm with a H3-type hysteresis loop, resulting in
2
O
3
without and with different
2
2
-1
2
O
3
2
O
3
2
-1
both high surface area (415 m g ) and narrow pore size distribution centered at ~3 nm
(
Figure S2). For meso-Al , a higher calcination temprature at 800 °C will lead to the
2
O
3
collapse of a portion of mesopores, as seen in Figure S3. Additionally, PEG, F127 and CTAB
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as templates also contribute to the formation of meopores smaller than 10 nm (Table 1). The
weight ratio of P123/Al precursor exhibites signifcant influence for the formation of
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mesopores. The N
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meos-Al synthesized with 0.4 g P123 displays a higher surface area of 644 m g and
2
O
3
narrower pore size distribution of ~5 nm (Figure 1a,b). It is nothworthy that the difference
between average pore size from Table 1 and pore size distribution from Figure 1 should be
ascribed to the limitation and accuracy of BJH model used. The transmission electron
microscopy (TEM) and scanning transmission electron microscopy (STEM) image of meso-
Al
2
3
O with 0.4g P123 as the template also reveal the existence of a sponge-like
nanoarchitecture with a large number of apparent mesopores (Figure 2a,b). A portion of
mesopores is marked by yellow circles from STEM-EDS image in Figure S4. The wide-angle
X-ray diffraction (XRD) result of meso-Al
shows an amorphous structure (Figure 3a, S5).
Table 1. Summary of pore parameter for different metal oxides calculated using N
2
O calcined at 400 °C with different templates
3
2
adsorption-desorption isotherms at 77 K.
2
-1
3
-1
Samples
S
BET (m g )
Pore volume (cm g )
Average pore size
(nm)
Al
2
O
3
246
644
257
0.32
0.75
0.16
~
meso-Al
2
O
3
(0.4 g P123)
(0.2 g PEG)
4.2
2.3
meso-Al
O
2 3
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meso-Al
2
O
3
(0.2 g CTAB)
(0.2 g F127)
(0.2 g P123)
380
329
415
115
143
133
178
168
176
129
198
0.26
0.24
0.34
0.11
0.27
0.14
0.24
0.15
0.18
0.13
0.24
~
meso-Al
meso-Al
2
2
O
3
3
~
O
2.6
2.8
4.9
7.2
5.8
3.3
6.3
3.9
4.8
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
CuO-Al
Mn -Al
Co -Al
Fe -Al
NiO-Al
MgO-Al
(CuNiFeCo)O
CuNiFeCoMg)O
O
2 3
3
O
4
2
O
3
3
O
4
O
2 3
2
O
3
O
2 3
2
O
3
O
2 3
x
-Al
O
2 3
(
x
-Al
O
2 3
aSpecific surface area calculated using the BET equation.
bSingle point adsorption total pore volume of pores.
cAverage pore diameter from the distribution maxima by resolving adsorption branch with BJH model.
Compared with monometallic oxide, binary metal oxides tend to possess wider
applications in catalysis. The traditional NHSG method for the production of metal oxides
often takes place in organic solvents and the corresponding pore size distribution is much
wide.[46,48] By the mechanochemical NHSG route in the solid-state, mesoporous binary metal
oxides (CuO-Al
2
O
3
, Mn
3
O
4
-Al
2
O
3
and Co
3
O
4
-Al
2
O ) with uniform pore size distribution
3
could be successfully synthesized with optimized PEG as the optimized template (Figure 1c-
h; Figure S6). In comparision to P123 and F127, PEG is a more safe, cheap and renewable
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
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5
5
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5
5
5
5
6
material. However, PEG has been used as soft templates with only limited success in
directing mesopores until now. XRD results reveal the clear crystalline structures of CuO
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
[
JCPDS 44-0706], Mn
Mn -Al and Co
crystalline size calculated by Scherrer equation are 15.9, 13.4 and 11.8 nm respectively.
Additionally, no diffraction peaks associated with Al can be found in three binary metal
oxides. Moreover, the diffraction peaks corresponding to NiO, MgO and Fe
3
O
4
[JCPDS 18-0803] and Co
3
O
4
[JCPDS 43-1003] in CuO-Al
2
O ,
3
3
O
4
2
O
3
3
O
4
-Al samples respectively (Figure 3a), and the corresponding
2
O
3
2
O
3
2
O
3
crystalline
samples
phase respectively were discovered in the NiO-Al
2
O
3
, MgO-Al
2
O
3
and Fe
2
O
3
-Al
2
O
3
respectively, as shown in XRD results (Figure S7).
500
2
a)meso-Al O3
b)
meso-Al O
c) CuO-Al O
2
3
d) CuO-Al O
2.4
2
3
2
3
4
3
2
00
00
00
60
40
0.4
0.2
0.0
1.8
1.2
0.6
0.0
2
0
0
100
0
0
.0 0.2 0.4 0.6 0.8 1.0
Relative Pressure (p/p0)
0
10
20
30
0.0 0.2 0.4 0.6 0.8 1.0
Relative Pressure (p/p0)
0
10
20
30
Pore width (nm)
Pore width (nm)
Pore width (nm)
2
1
00
50
0.6
e) Mn O -Al O
f) Mn O -Al O
3
4
2
3
3
4
2
3
90
g) Co O -Al O
h) Co O -Al O
3
4
2
3
3
4
2
3
0.4
0.4
0.2
0.0
60
100
0.2
0.0
3
0
0
5
0
0
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.2 0.4 0.6 0.8 1.0
0
10
20
30
0
10
20
30
Relative Pressure (p/p°)
Pore width (nm)
Relative Pressure (p/p°)
Pore width (nm)
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Figure 1. N
2
adsorption-desorption isotherms (a,c,e,g) and the corresponding pore size
distribution (b,d,f,h) of meso-Al
2
O
3
, CuO-Al
2
O
3
, Mn
3
O
4
-Al
2
O
3
and Co
3
O
4
-Al
2
O
3
synthesized using mechanochemical NHSG method.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The surface area and pore structure of CuO-Al
were examined with N
samples show representative type-IV curves with clear capillary condensation steps between
the relative pressure (P/P ) of 0.4−0.8, underscoring the presence of rich mesopores. The
specific surface areas of CuO-Al , Mn -Al and Co -Al are 115, 144 and 133
m g , respectively (Table 1). More importantly, the pore size distributions by resolving the
adsorption branch of N isotherms with Barrett-Joyner-Halenda (BJH) method centred at 4,
6 and 7 nm for CuO-Al , Mn -Al and Co -Al
respectively (Figure 1d,f,h).[49]
Moreover, N adsorption and pore size distribution results of NiO-Al , MgO-Al and
Fe -Al samples also reveal the existence of abundant mesoporous structure (Table 1;
2
O
3
, Mn
3
O
4
-Al
2
O
3
and Co
O
3 4
-Al
2
O
3
2
adsorption-desorption isotherm measurements (Figure 1c,e,g). All
0
2
O
3
3
O
4
O
2 3
3
O
4
2
O
3
2
-1
2
O
2 3
3
O
4
2
O
3
3
O
4
2
O
3
2
2
O
3
2
O
3
2
O
3
2
O
3
Figure S8). These obained abundant and uniform mesopores in binary metal oxides should
be ascribed to solid-state self-assembly behavior of PEG micelles and different metal ions,
confining NHSG process within the periphery of micelles. The results indicate that the
mechanochemical NHSG route has the traits of universality for the preparation of a variety
of binary metal oxides with uniform mesopore distributions. Additionally, the obtained
surface area for both meso-Al
2
3
O and binary metal oxides are comparable to the previously
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
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5
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5
5
5
5
6
reported mesoporous alumina and metal containing mesoporous alumina synthesized by
conventional wet chemistry[24,50]
.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Mn -Al is then chosen as a representative to investigate the pore structure of
3
O
4
2
O
3
binary metal oxides in a more tangible manner. Wormhole architectures with rich mesopores
side by side are observed throughout the material backbone in the range of several hundred
nanometers (Figure 2c,d). Furthermore, energy-dispersive X-ray spectroscopy (EDS)
mapping results of representative binary Mn
3
O
4
-Al
2
3
O sample (Figure 2e-g; Figure S9) show
the well distribution of Mn, Al and O, suggesting that the mesoporous structure is fabricated
by the uniform mixture of Mn
3
O
4
and Al
samples in Figure S10,11 also display existence of
) and Al
2
O . Additionally, TEM and EDS-mapping results
3
for CuO-Al and Co -Al
2
O
3
3
O
4
2
O
3
mesopores and unifrom mixture of CuO (Co
3
O
4
2
3
O .
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1
1
1
1
1
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1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2. TEM a) and STEM-HADDF b) images of meso-Al
2
3
O ; STEM-HADDF images
(c,d) of Mn
3
O
4
-Al
2
3
O and the corresponding element mapping signals e) Al, f) Mn and g) O.
Interestingly, the XRD result of (CuNiFeCo)O
x
-Al
2
3
O sample synthesized by the
same method does not show the characteristic diffraction peaks of CuO, Fe
2
O
3
or Co
3
O
4
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1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
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(
Figure S12). The XRD pattern of (CuNiFeCo)O
which is close to 37.2 (111), 43.3 (012), 62.8 (110) and 75.4 (113) reflections of cubic NiO
JCPDS 65-2901). Above results suggest that Cu, Fe and Co may be incorporated into the
sublattice of NiO to form a solid solution. STEM-HAADF images of (CuNiFeCo)O -Al
sample reveal the aggregations of hybrid crystallites with a high degree of interstitial porosity
Figure 4a,b). EDS-mapping results show the highly uniform distribution of four metal
species (Cu, Ni, Fe and Co), further suggesting the existence of (CuNiFeCo)O solid solution
Figure S13). The pore nature of (CuNiFeCo)O -Al sample was then evaluated via N
adsorption-desorption measurement at 77 K. A type-IV isotherm with an H type hysteresis
loop between the relative P/P of 0.4 to 0.7 can be observed, leading to a surface area of 129
x
-Al
2
O
3
reveals its crystalline structure,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
x
2
O
3
(
x
(
x
2
O
3
2
3
0
2
-1
m g (Table 1) with narrow pore size distribution (~4 nm) (Figure 5a,b). The molar ratio
of Al:(CuNiFeCo) obtained from ICP-AES result is 1.9, similar with 1.5 determined from
EDS result in Figure S13.
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100
a)
b)
Al O
2
3
8
0
CuO-Al O
2
3
meso-Al O
2
3
CuO-Al O
2
3
CuO
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
60
Co O -Al O
3
Mn O -Al O
3
4
2
3
4
2
3
Mn O4
3
Mn O -Al O
3
3
4
2
(CuNiFeCoMg)O -Al O
x
2
3
4
2
0
0
0
Co O -Al O
3
4
2
3
Co O4
3
(
CuNiFeCoMg)O -Al O
x 2
3
NiO
20
30
40
50
60
70
80
5
0
100
150
200
250
300
2
Temperature (C)
74.6 eV
2+
Co
Al 2p
c)
d)
7
81 eV
Ni 3p
2+
Cu 3p
Co satellite
7
86.5 eV
(
CuNiFeCoMg)O -Al O
x 2
3
meso-Al O3
2
84
82 80 78 76 74 72 70 68 66
Binding energy (eV)
810
805
800
795
790
785
780
775
Binding energy (eV)
Figure 3. XRD results of a) meso-Al
b) CO oxidation activity over different catalysts; Normalized XPS spectrums of c) Al 2p and
d) Co 2p transitions for meso-Al (peak intensity for Al 2p is reduced) and
2
O
3
, binary metal oxides and (CuNiFeCoMg)O
x
-Al
2
3
O ;
2
O
3
(
CuNiFeCoMg)O
x
-Al
2
O
3
samples.
Crystalline high-entropy metal oxides (normally containing five or more metal
species), as a new class of material, have attracted great interest because of their unique
structure and potential applications.[51,52] To date, the state-of-art high-entropy metal oxides
only possess very limited surface area (e.g., <30 m g-1)[53,54], because their porous structures
2
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1
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1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
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6
would collapse during the removal of templates and the crystalllization process, which limites
their performance in catalysis. To synthesize mesoporous high entropy metal-Al mixed
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
oxides, Mg was chosen as the fifth metal element for the preparation of (CuNiFeCoMg)O
x
-
Al . The basis for the choice of these metals is their similar atomic radius. The
2
O
3
corresponding XRD patterns are displayed (Figure 3a). It is reassuring that when Mg is used
as the fifth metal, the diffraction peaks become similar with the shift of diffraction peaks of
cubic crystalline NiO. The diffraction peaks belonging to CuO, Fe
2
O
3
, Co
O
3 4
and Mn
3
4
O in
binary metal-Al oxides disappeared accidentally, suggesting the formation of single high-
entropy phase). The crystalline size of high-entropy phase is approximately 11.6 nm
determined by Scherrer equation. Moreover, SEM-EDS results reveal that five elements (Cu,
Ni, Fe, Co and Mg) signal are simultaneous discovered in selected regions (Figure 4e-l). The
obtained results suggested that the high-entropy metal oxides (CuNiFeCoMg)O
x
should be
formed and mixed uniformly with Al to form a mesoporous complex. We deduced that
2
O
3
the mechanochemical process together with calcination step in are the major reason for the
formation of solid solution.
The metal valence is illustrated by X-ray photoelectron spectroscopy (XPS) analysis,
and the results are shown in Figure 3c,d and Fiugure S14. The Al 2p XPS spectra in Figure
3
c show the primary peak located at 74.6 eV for both meso-Al
2
O
3
and (CuNiFeCoMg)O
x
-
Al
2
O
3
samples, suggesting the existence of Al-O bonds in Al3+ (often known as Al
2
O
3
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species). The Cu 2p spectrum in Figure S14a can be fitted into four peaks, corresponding to
Cu 2p3/2 (933.0 eV) and Cu 2p1/2 (953.0 eV) and their satellite peaks. These characteristics
match well with standard peaks of CuO. The similar results in Ni 2p spectrum can identify
the possible existence of NiO (Figure S14b)[55]. The main peak from Mg 2p spectrum in
Figure S14c is located around 50 eV, but valence state of Mg is difficult to be determined
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
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4
4
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0
2+
due to the close peak position of different state (Mg and Mg ). However, Mg should exist
0
in MgO form considering that the synthesis process is difficult to generate Mg . For Co 2p
spectrum in Figure 3d, the peaks at 781 eV together with strong satellite at 786.5 eV indicate
the existence of Co , since the signature of Co have no satellite peak[56]. The Fe 2p
2
+
3+
2
+
3+
spectrum in Figure S14d suggests the co-existence of Fe and Fe .
STEM-HAADF images of (CuNiFeCoMg)O -Al sample suggest the existence of
rich mesoporosity marked with small yellow circles (Figure 4c-d), which is also revealed by
SEM images in Figure S15. The pore nature of (CuNiFeCoMg)O -Al sample was then
evaluated by N adsorption-desorption measurement at 77 K. A type-IV isotherm with steep
adsorption peaks between the relative pressure (P/P ) of 0.2−0.8, revealing the abundant
mesoporous structure (Figure 5c,d). It offered a surface area up to 198 m g , and the relative
lower surface area compared to meso-Al should be attributed to the blockage and collapse
x
2
O
3
x
2
O
3
2
0
2
-1
2
O
3
of pore channels during the crystallization process of high-entropy metal oxides. The molar
ratio of Al:(CuNiFeCoMg) obtained from ICP-AES result is 1.3.
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Figure 4. STEM (a) and SEM (b) images of (CuNiFeCo)O
x
-Al
2
O
3
; STEM (c,d) images of
-Al and the
(CuNiFeCoMg)O
x
-Al
2
O
3
; SEM-EDS image (e) of (CuNiFeCoMg)O
x
2
O
3
corresponding element mapping signals f) Al, g) Cu, h) Ni, i) Fe, j) Co, k) Mg, l) O.
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Pore width (nm)
0
.6
.4
80 a)
b)
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30
Relative Pressure (p/p°)
Pore width (nm)
Pore width (nm)
0.5
1
1
60 c)
d)
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0.3
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0.0
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Relative Pressure (p/p°)
Pore width (nm)
Figure 5. N
(CuNiFeCo)O
mechanochemical NHSG method.
2
adsorption-desorption isotherms and the corresponding pore size distribution of
x
-Al (a,b) and (CuNiFeCoMg)O -Al (c,d) synthesized using
2
O
3
x
2
O
3
Subsequently, the catalytic activities of different mesoporous metal oxides are
evaluated for CO oxidation (Figure 3b). The T100 values (temperature for 100% CO
conversion) are 220 °C (CuO-Al
2
O
3
), 270 °C (Co
3
O
4
-Al
2
O
3
) and 260 °C ((CuNiFeCoMg)O
exhibits much worse activity for
phase. And
catalyst become active at 120 °C and CO was completely converted
x
-
Al ), respectively. Among these catalysts, Mn
2
O
3
3
O
4
-Al
2
O
3
the oxidation of CO, which may be attributed to the relatively inactive of Mn
mesoporous CuO-Al
3
O
4
2
O
3
at 220 °C, which are superior to most catalysts especially for Cu-Al mixed oxides prepared
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using traditional impregnation and precipitation method as shown in Table S2.[57-59]
Moreover, both CuO-Al
2
O
3
and (CuNiFeCoMg)O
x
-Al
2
3
O catalysts exhibted remarble
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stability even after used for 48 h (Figure S16a,b). Considering the facile, fast and solvent-
free preparation method, these mesoporous mixed oxides may have great potential
applications in industrial catalysis. As shown in Figure S17, XRD results of fresh and spent
CuO-Al
2
O
3
and (CuNiFeCoMg)O
x
-Al
2
3
O catalysts display similar diffraction peaks and
crystallite size. Additionally, temperature programmed desorption (TPD) of CO in Figure
S18 reveals similar CO desorption peak area, indicating the dispersion of metal active species
does not decrease remarkably after use.
The sulfur tolerance is of great importance in real-world catalysis. However, CO
oxidation activity of most catalysts went down remarkably in the presence of SO
affecting the process of industrialization. Therefore, we compared the performance of SO
resistance over CuO-Al and (CuNiFeCoMg)O -Al catalysts. Before the catalytic
activity test, each catalyst was treated in 1000 ppm SO
activity of CuO-Al decreased remarkably after SO
noteworthy that there is a negligible effect of SO
Figure S19b). The decreased activity of CuO-Al
2
, greatly
2
2
O
3
x
2
O
3
2
at 280 °C for 5 h. The catalytic
treatment (Figure S19a). It is
2
O
3
2
2
on (CuNiFeCoMg)O
x
-Al
2
3
O catalyst
(
2
O
3
is attributed to the formation of sulfites,
-1
evidenced by the feature in FTIR spectra around 1100 cm (Figure S10).[60-61] However, it is
clear that the formation of crystalline high-entropy metal oxides protect iteslf from SO
2
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poisoning, contributing to the superior SO
indicates that the (CuNiFeCoMg)O -Al
configurational entropy, preventing itself from being sulfurated.[51]
2
tolerance compared with CuO-Al
2
O . This result
3
x
2
O
3
catalyst may be stablized by the relative higher
1
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. Conclusions
In summary, an efficient, fast and facile route for the synthesis of mesoporous Al
2
O
3
and its metal oxide hybrids has been developed based on mechanochemical non-hydrolytic
2
-1
method. The obtained meso-Al
2
3
O shows a record high surface area (~644 m g ) and
unifrom pore size (~5 nm). Moreover, mesoporous metal-aluminum oxide and four or five
metal solid solution-aluminum oxide hybrids with rich mesoporous structure and single cubic
crstalline phase was discovered. More importantly, this mechanochemical method does not
need solvents and can shorten the pore filling process from 1 ~ 3 days to 60 min. The obtained
mesoporous mixed metal oxide (such as (CuNiFeCoMg)O
x
-Al
2
O
3
) exhibit not only an
tolerance. This
exceptional catalytic performance in CO oxidation, but also the superior SO
2
mechanochemical NHSG method may open the door to the scalable preparation of a new
class of metal oxide catalysts.
Supporting Information
Additional results of characterization results (XRD, SEM, TEM, N
2
adsorption-desorption,
CO-TPD, ICP-AES and FTIR)
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Acknowledgments
S. Dai was supported by the Division of Chemical Sciences, Geosciences, and
Biosciences, Office of Basic Energy Sciences, US Department of Energy. J. Fu was supported
by the National Natural Science Foundation of China (No. 21436007, 21706228), the
Zhejiang Provincial Natural Science Foundation of China (No. LR17B060002). Z. Zhang
thanks the China Scholarship Council for financial support as a joint PhD student. P. F. Z.
acknowledges Shanghai Pujiang Program (Grant No.17PJ1403500), Thousand Talent
Program, National Natural Science Foundation of China (Grant No. 21776174), and
the Open Foundation of the State Key Laboratory of Ocean Engineering (Shanghai
Jiao Tong University of China) (No. 1809) for the support.
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