A Principle for Highly Active Metal Oxide Catalysts via NaCl-Based Solid Solution

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Article

A Principle for Highly Active

Metal Oxide Catalysts via

NaCl-Based Solid Solution

Yuan Shu, Hao Chen, Nanqing Chen, Xiaolan Duan, Pengfei Zhang,¹^,⁶^,* Shize Yang,⁴^,⁵

1

3

1

2

2,3

Zhenghong Bao, Zili Wu, and Sheng Dai

SUMMARY

The Bigger Picture

Toward the preparation of industrial metal oxide catalysts, sacriﬁ-

cial organic templates, excessive solvents, complex impregnation,

and drying steps are generally required. Here, we report a versatile

rule for the simple synthesis of highly porous metal oxides with well-

dispersed noble metal species. Porous metal oxides (Co O , Fe O ,

Transition metal oxides (TMOs)

are the most important catalysts in

industrial chemistry. Tailoring the

porosity of TMOs and the

dispersion of active noble metal

species are two strategies to

enhance catalytic performance.

However, the method of

3

4

x

y

and Cr O ) are obtained with some surface areas (e.g., Cr O :

2

3

2

3

2

ꢀ1

2

24 m $g ) beyond the record value. Surprisingly, small noble

metal nanoparticles (e.g., Pd: 3.1 and Pt: 3.2 nm) could be incorpo-

rated by this solid-state process simultaneously. Corresponding

Rh-Co O , Pd-Fe O , and Pt-Cr O exhibit excellent performance:

simultaneously producing

porosity and high dispersion is

quite scarce. Here, we propose a

principle that NaCl-based solid

solution could serve as a versatile

platform to control both porosity

and dispersion within TMO

3

4

x

y

2

3

ꢂ

CH₄combustion (T₉₀= ꢁ360 C and thermal stability: >100 h at

ꢂ

6

80 C), hydrogenation of nitrobenzene and derivatives (turnover

4

number [TON] = 2.49 3 10 , 300 mmol per run), and reversed water

gas shift (RWGS) reaction (44% CO₂conversion with ꢁ98% CO

ꢂ

selectivity and thermal stability: >100 h at 500 C), respectively.

Therefore, current principle via a NaCl-based solid solution could

provide a solid-state, fast, and efﬁcient route for processing metal

oxide catalysts.

catalysts. The essence of this

strategy lies in the ion-sharing

talent of NaCl as a salt matrix by

mechanochemistry, which

promotes the high distribution of

transition-metal precursors and

noble metal species on NaCl

crystalline. Besides, removing

NaCl also releases pores. The

corresponding TMO catalysts

exhibit quite high catalytic

INTRODUCTION

Metal-oxide-based catalysts are of great importance in chemical industry. Toward

the design of high-performance catalysts, the porous structure and dispersion

of active phase are normally two key factors to be considered. It is well known that

a high surface area is beneﬁcial to the exposure of active sites, the adsorption,

1

–3

mass transfer, and desorption of the reactants, and so on.

For example, porous

manganese oxide catalyst showed excellent CH

4

combustion activity with T10 (tem-

ꢂ

performance in redox reaction.

perature at which 10% CH

4

is converted) around 250 C, much lower than that

ꢂ

4

(

358 C) by commercial bulk Mn

2

O

3

material. The dispersion of active phase is the

other issue, mainly related to the size of noble metal nanoparticles (NPs). When

the size is reduced to 1ꢁ3 nm, due to the integrated effects of increased low-coor-

dinated surface atoms, surface relaxation, and quantum size effects, the perfor-

5

mance of those catalysts would be generally improved. For instance, Au NPs

with sizes between 0.5ꢁ5 nm showed high catalytic performance in CO combustion,

6

,7

but larger Au NPs are relatively inactive in the same process.

Controlling porous structure of metal oxides and dispersion of noble metal NPs

usually require several steps. In general, porous metal oxides as supports or catalysts

are synthesized by precipitation, solution combustion, or sol-gel methods to

produce metal oxides with interstitial porosity. Meanwhile, metal oxides with

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1

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well-deﬁned and interconnected mesopores could be prepared by soft- or hard-

8

–11

templating methods.

Based on self-assembly of organic surfactant aggregations

and inorganic salt species, soft-templating technology greatly promotes the chem-

istry of mesoporous metal oxides, whereas several issues still remain, such as: exces-

sive use of organic solvents, the long aging process from several days to a month,

1

2

and the sacriﬁcial behavior of surfactant molecules. Meanwhile, hard-templating

strategy has been well-mastered in the past few decades and provides a general so-

8

,13,14

lution to prepare mesoporous metal oxides.

Santa Barbara amorphous-15 [SBA-15], mobil composition of matter number 41

However, speciﬁc templates (e.g.,

[

MCM-41], and Korea Advanced Institute of Science and Technology number 6

KIT-6]) as matrixes are necessary, and the removal of silica templates by hazardous

chemicals (e.g., concentrated NaOH) adds an elaborate procedure, prompting re-

searchers to explore alternative strategies. After acquiring porous supports, impreg-

nation method (IMP) and deposition-precipitation method (DP) are typical methods

1

5–17

to load and disperse noble metal NPs.

In fact, the above-mentioned methods

are rather successful in the construction of famous industrial catalysts, yet several is-

sues still have not been well resolved. To date, the direct preparation of metal oxide

catalysts with plentiful pores and well-dispersed noble metal species is highly desir-

able—from the standpoint of both fundamental and industrial catalysis.

As a powerful synthesis tool, mechanochemistry has shown attractive efﬁciency

1

8–20

21

in the synthesis of zeolites,

single-atom catalysts (SACs), organic mole-

2

2–25

26

27

28

29–32

cules,

MOF, COF, carbon, and supported metal oxides.

Based on

3

3–35

our previous works on porous metal oxides by solid grinding method

and

we questioned whether

NaCl can serve as a salt template to direct porosity and at the same time dilute noble

2

8,36–40

inspired by the excellent solid solubility of NaCl,

2

+

metal ions (e.g., Pd ) in solid state. More encouragingly, a three-dimensional (3D)

printing strategy of utilizing NaCl together with surfactants and solvents has already

4

1

been reported for the fabrication of Mg scaffolds with well-controlled porosity. In

this research, via a versatile NaCl-based solid solution, we reported a general prin-

ciple for a simple preparation of highly porous metal oxides and related noble metal

catalysts. Current synthesis is in fact different from the traditional molten salt

4

2–44

method,

as reﬂected by the following features: the calcination temperature

ꢂ

(

300 C) is far below the melting point of NaCl (ꢁ801 C) or related freezing points

e.g., freezing points of NaCl-CoCl

2

and NaCl-FeCl

45

3

at NaCl/(MCl

x

+NaCl) (mol/

ꢂ

mol) = 0.81 were 690 C and 772 C, respectively. ). The NaCl-MCl

x

solid solution

is formed by mechanochemically grinding in solid state. After calcination, and

removing and recycling NaCl by washing in water, a series of porous metal oxides

(

Fe

x

O

y

, Cr

2

O

3

, and Co

3 4

O ) were successfully synthesized, and the special surface

2

46

area (SSA) of Cr

2

O

3

(224 m /g) is a record value. Surprisingly, a family of bulky

¹School of Chemistry and Chemical Engineering,

Shanghai Jiao Tong University, Shanghai 200240,

China

precious metal precursors could be processed into nano- or sub-nano scale by

this solid state NaCl-diluting approach only. The preferred porosity and dispersion

of those catalysts (Pd-Fe

x

O

y

, Pt-Cr

2

O

3

, and Rh-Co

3

O

4

) endow them with excellent

in CH combus-

in nitrobenzene hydrogena-

²Chemical Sciences Division, Oak Ridge National

Laboratory, Oak Ridge, TN 37831, USA

activity and stability in various redox reactions, including: Rh-Co

3

O

4

ꢂ

tion (T90 = 360 C, Stability at 680 C >100 h), Pd-Fe

x

O

y

3

Department of Chemistry, University of

Tennessee, Knoxville, TN 37996, USA

4

tion (TON = 2.49 3 10 , 300 mmol Per Run), and Pt-Cr

2

O

3

in CO

2

hydrogenation

ꢂ

4

Materials Science and Technology Division, Oak

(

conversion [Conv]: 44%, selectivity [Sel.] for CO: 98%; Stability at 500 C >100 h).

Ridge National Laboratory, Oak Ridge, TN 37831,

USA

⁵Brookhaven National Laboratory, Upton, New

York, NY 11973, USA

RESULTS AND DISCUSSION

Synthesis and Characterization of Mesoporous Metal Oxides

⁶Lead Contact

To verify the hypothesis if alkali metal-based solid solution could be used as a

porosity-directing additive, we initially investigated the solubility of cobalt chloride

*

Correspondence: chemistryzpf@sjtu.edu.cn

2

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Figure 1. XRD Patterns of Salt Mixtures

(

A) XRD patterns of the mixture by ball milling CoCl

B) XRD patterns of the intermediate products.

2

and alkali metal salts (NaCl, KCl, LiCl, or NaBr).

Step a. ball milling NaCl and CoCl

2

; Step b. ball milling the mixture with NaOH; Step c. calcination

in air; Step d. recycling NaCl by washing in water.

(

CoCl

ionic radii. Typically, after ball milling of salt mixtures (e.g., CoCl

) for 0.5 h, the intermediate product was immediately characterized by X-ray

2

) in several alkali metal salts (e.g., KCl, NaCl, LiCl, and NaBr) with different

2

-NaCl in ꢁ1/1wt

%

˚

diffraction (XRD). Interestingly, when the solvent was NaCl (a = 5.67 A), the XRD ex-

ꢂ

hibited a group of typical peaks (27.3 , 31.7 , 45.5 , and 56.5 ) for NaCl (Joint Com-

mittee on Powder Diffraction Standards Number [JCPDS NO.] 05-0628). No XRD

2

+

2

peaks relating to CoCl were detected, indicating that Co cations were well-

dispersed in the solid NaCl lattice (See Figure 1A). On the contrary, with KCl as

the diluter, the XRD pattern showed multi peaks for both KCl (JCPDS NO.

4

1-1476) and CoCl

2

(JCPDS NO. 72-2408). The larger lattice parameter (a =

˚

6

.29 A) of KCl might have led to this mismatching behavior of KCl-CoCl

2

˚

crystals. Similar observations occurred in the cases of CoCl -LiCl (a = 5.14 A) and

2

˚

2

-NaBr (a = 5.97 A), whose diffraction patterns evidently deviated from the

CoCl

precursors. Among the salts studied above, NaCl seems like a suitable solid solvent

for diluting and restructuring CoCl

2

.

The mechanochemical synthesis of porous metal oxides via NaCl-based solid solu-

tion was shown in Scheme 1, and each step was carefully studied by XRD (Figure 1B).

2

At the beginning, the mixing of NaCl (0.5–2 g) and CoCl (0.475 g) was promoted

2

+

by ball milling. After the homogeneous dispersion of Co cations, stoichiometric

NaOH as the oxygen donor was added into the mixture. Because of the strong

2

+

ꢀ

ꢀ15 47

Coulomb interaction between Co and OH (Ksp-a-Co(OH)

2

= 1.09 310 ),

the selective transformation of Co to the corresponding hydroxide occurred,

which was evidenced by the presence of characteristic peaks for Co(OH) (JCPDS

NO. 74-1057). The weak XRD intensity of Co(OH) suggested that the solid state

coordination between Co and OH might undertake around the NaCl crystalline

matrix. Interestingly, after calcination of Co(OH) into Co , strong XRD peaks

for NaCl with poor responses for Co were observed in the mixture, revealing

the good dispersion of Co species. Finally, the NaCl template was removed

2

+

2

+

ꢀ

2

3 4

O

3

O

4

3

O

4

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Scheme 1. General Synthesis of Porous Metal Oxides and Corresponding Supported Metal

Oxides through NaCl-Based Solid Solution

(

A) Ball milling a mixture of NaCl particles and transition metal precursors in 0.5 h.

B) Ball milling the intermediate together with NaOH in 0.5 h.

C) Calcination in air.

D) Recycling NaCl by washing in water.

The bottom part shows three as-made catalysts and their application in model redox reactions.

and recycled by washing in water, accompanied by the release of porosity. The XRD

3 4

pattern of resulting Co O sample (See Figures 1B and S1) showed ﬁve broad peaks

ꢂ

at 31.1 , 36.6 , 49.1 , 59.3 , and 65.1 , corresponding to the (111), (220), (311),

400), (511), and (440) planes of cubic phase Co (JCPDS NO. 43-1003), respec-

tively. The average crystal size of the Co estimated by Scherrer’s equation was

2.1 nm.

(

3 4

O

3

O

4

1

The SSAs of Co

NaBr, and NaNO

From Figure 2A; Table 1, we can see that the type of dilute salts has an important

inﬂuence on the SSA value. The NaCl-derived Co (NaCl) sample had a good

SSA of 110 m /g, meanwhile its sorption curve exhibited a typical type IV isotherm

with a H -type hysteresis loop between relative pressure P/P

= ꢁ0.5–1.0, implying

3

O

4

samples synthesized with different salts (KCl, NaCl, LiCl, CsCl,

3

) were evaluated by nitrogen sorption measurements at 77 K.

3

O

4

2

3

0

4

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Figure 2. N

2

Adsorption Curves of Porous Metal Oxides

sorption isotherms (77K) of Co samples synthesized by different solid salts.

sorption isotherms (77K) of Cr and Fe obtained by NaCl method.

(

A) N

B) N

2

3 4

O

2

O

3

x y

O

Synthetic condition: transition metal salts 2 mmol, NaCl 0.5 g, stoichiometric NaOH 4 mmol or

ꢂ

6

mmol, and calcination temperature 300 C.

the formation of interstitial mesopores. However, control synthesis of Co

3 4

O without

NaCl resulted in a decrease in SSA calculated by Brunner-Emmet-Teller (BET)

2

method (Co

3

O

4

-blank with 70 m /g). On the other hand, when LiCl, KCl, CsCl, or

samples decreased to 74,

7, 37, and 31 m /g, respectively. This trend somewhat supports the observation

NaBr were used as the solid solvent, the SSAs of Co

3

O

4

2

6

2

by XRD, and the formation of CoCl -NaCl solid solution could contribute more

porosity into Co

ate effect on SSA of Co

followed the sequence of Co

3

O

4

. In addition, the amount of NaCl additive (0–4 g) had a moder-

3

O

4

, and the BET SSAs of Co by different NaCl dosages

3 4

O

2

3

O

4

-2 g: 116 m /g, Co

3

O

4

-0.5 g: 110 m /g, Co

-0 g: 70 m /g. Therefore, 0.5 g NaCl was selected as the appro-

priate dosage to transforming 2 mmol CoCl (0.475 g) into porous Co . In fact, the

dosage of NaCl (0.5 g) is low compared with the molten salt method, during which

3 4

O -4 g:

2

8

3 4

6 m /g, and Co O

2

3 4

O

4

2–44

salt additives with 10–20 dosages by weight were generally needed.

By the way,

ꢂ

the current calcination temperature (300 C) was lower than the freezing point of

ꢂ

45

NaCl-CoCl

2

at NaCl/(CoCl

2

+NaCl) (mol/mol) = 0.81 (690 C), and thus the current

ꢂ

process was undergone in solid state. The calcination temperature (300 C) is similar

to those by precipitation method, impregnation method and deposition-precipita-

tion method, thus the energy consumption of this method is comparable to that

of other traditional methods.

In order to study the scope of this method, two other metal oxides (Fe

x y 2 3

O and Cr O )

were prepared by using NaCl as the salt template. The crystal phases of Fe

x

y

O and

Cr

2

O

3

were conﬁrmed by XRD (See Figure S1). The Fe

x

O

y

sample exhibited eight

ꢂ

peaks at 18.3 , 24.1 , 30.1 , 33.1 , 35.4 , 53.4 , 56.9 , and 62.5 , among which those

ꢂ

2 3

peaks located at 24.1 and 33.1 belonged to (104) and (012) planes of Fe O

(

JCPDS NO.89-0597), and the other peaks corresponded to (111), (220), (311),

422), (511), and (440) planes of Fe (JCPDS NO.79-0419), respectively. Thus,

and Fe . The Cr sample showed six clear

peaks which were assigned to the (104), (110), (202), (024), (214), and (300) planes of

Cr (JCPDS NO.84-0312). The average crystal sizes of Fe and Cr by Scher-

(

3

O

4

the sample was a mixture of Fe

2

O

3

O

4

2 3

O

2

O

3

x

O

y

2 3

O

rer’s equation were 21 and 13 nm, implying that Fe and Cr precursors also could be

well-dispersed on NaCl solid matrix.

By the N

2

sorption isotherms at 77 K, the SSAs of these metal oxides were quite high

2

(

Fe : 100 m /g, Cr

x

O

y

2

O

3

: 224 m /g), whereas those of Fe

x

O

y

and Cr

-blank:73 m /g), further

sample

2 3

O obtained

2

without NaCl were much lower (Fe

x

O

y

-blank:53 m /g, Cr

2

O

3

2 3

proving the importance of a NaCl template (Table 1). The BET SSA of Cr O

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Table 1. BET Surface Areas of Corresponding Metal Oxides

2

a

Sample

Salt

Source

Noble Metal

N/A

SSA (m /g)

Co

3

Co

3

Co

3

Co

3

Co

3

Co

3

Co

3

Co

3

Co

3

O

4

-Blank

N/A

CoCl

2

70

NaCl (0.5 g)

LiCl (0.5 g)

KCl (0.5 g)

NaBr (0.5 g)

CsCl (0.5 g)

N/A

110

67

N/A

74

N/A

31

-CsCl

N/A

37

b

4-NO

3

NaNO

3

(0.5 g)

N/A

99

4

-2g

-4g

NaCl (2 g)

NaCl (4 g)

N/A

116

86

N/A

Fe

x

O

y

-Blank

FeCl

3

N/A

53

x

O

NaCl ( 0.5 g)

3

N/A

100

249

73

b

x

-NO

3

NaNO

N/A

3

(0.5 g)

3

N/A

Cr

2

Cr

2

Cr

2

3

-Blank

CrCl

3

N/A

NaCl (0.5 g)

NaNO (0.5 g)

N/A

224

191

142

172

111

b

-NO

3

N/A

Rh-Co

Pd-Fe

Pt-Cr

3

O

4

NaCl (0.5 g)

CoCl

FeCl

2

RhCl

3

x

O

y

3

PdCl

2

O

3

CrCl

3

2 6

H PtCl

a

SSA calculated by using the BET equation.

b

ꢂ

The NaNO template was decomposed during calcination at 300 C.

3

2

(

224 m /g) is a record value to the best of our knowledge (Cr

2

O

3

obtained

by using KIT-6 or SBA-15 as

synthesized by CTAB as template:

also functioned well for generating porosity

-NO , and Cr -NO were 99, 249, and

2

48

by pyrolysis of MIL-101-Cr: 77.4 m /g, Cr

2

O

3

2

49,50

hard-templating: 70ꢁ 92 m /g,

2 3

Cr O

2

51

1

25–143 m /g ). Moreover, NaNO

3

into metal oxides (Co

3

O

4

-NO

3^ꢀ

3

, Fe

x

O

y

3

2

O

3

2

1

91 m /g). However, NO

ions would be decomposed during calcination

ꢂ

(

300 C), therefore NaNO

3

cannot be recycled and reused. Based on those results,

NaCl seems like a better choice for the synthesis of porous metal oxide.

To study the macroscopic morphology of those metal oxide catalysts, we carefully

performed scanning electron microscopy (SEM) characterization (see Figure 3).

3 4 2 3 x y

Those metal oxides (Co O , Cr O , and Fe O ) displayed coral reef-like structure,

and apparent interstitial pores could be observed between aggregated particles.

This morphology is somewhat similar with porous materials by ionothermal synthe-

5

2–54

sis,

during which metal oxides would form around NaCl solid solution and

porous aggregations with small particles are left after the removal of NaCl.

2 3

Due to the excellent value in SSA, material morphology of the Cr O sample was

further observed by scanning transmission electron micrographs (STEM). As seen

from Figure 4A, a sponge-like nanostructure with plentiful and continuous pores

can be observed throughout the bulky aggregation in several micrometers. More

clear views of the porous structure were observed by combining high-resolution

high angle annual dark ﬁeld (HAADF) image and corresponding bright ﬁeld (BF)

image (See Figure S2). The apparent mesopores with diameters around 3–15 nm

could be found side by side, which roughly coincided with the pore size distribution

by the Barret-Joyner-Halends (BJH) model using N

2

adsorption branch (See

6

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3 4 x y 2 3

Figure 3. SEM Images of Porous Metal Oxides (Co O , Fe O , and Cr O )

Figure S3). The irregular accretion of tiny Cr

2 3

O NPs mainly contributed to the for-

mation of porosity. The crystalline of Cr was further checked by high-resolution

2

O

3

transmission electron microscopy (HRTEM) (See Figure 4B), and the found spacings

of lattice fringes were 0.23 and 0.26 nm, which agreed well with the (113) and (104)

4

9

2 3

crystal planes of Cr O , respectively. These data demonstrate that current strategy

using NaCl-based solid solution can produce porous metal oxides with high surface

areas.

Encouraged by the porosity-directing ability of this method, NaCl-based solid solu-

tion was extended to dilute noble metal species. The initial solid solution was formed

by ball grinding NaCl, transitional metal chlorides, and noble metal chlorides.

Several catalysts (Rh-Co

tion and the removal of NaCl. XRD was used to characterize the crystalline phases of

Pd, Rh, and Pt on Pd-Fe , Rh-Co , and Pt-Cr , respectively. With Pt-Cr as

an example, those peaks can be well-indexed to typical cubic phase of Cr (See

3 4 x y 2 3

O , Pd-Fe O , and Pt-Cr O ) can be obtained after calcina-

x

O

y

3

O

4

2

O

3

2 3

O

2

O

3

Figure S4). Although Pt content by inductively coupled plasma atomic emission spec-

trometry (ICP-AES) was quite high (9.4 wt %), no clear diffraction peaks relating to Pt

species were observed in the sample of Pt-Cr

Pt NPs on the Cr support. The morphology and size of Pt species were further

characterized by STEM-HAADF images. According to the Figures 4C, 4D, and S5,

the Pt NPs were homogeneously anchored over the Cr support in the domain

2 3

O , indicating the good dispersion of

2

O

3

2

O

3

of several hundred nanometers and the average size of Pt NPs was 3.2 nm. However,

further dilution of precious metal chloride by increasing the proportion of NaCl did

not effectively reduce the size of Pt NPs (ꢁ3.5 nm) (See Figure S6).

For Pd-Fe

x y 3 4

O and Rh-Co O catalysts, the typical peaks of Pd and Rh species

were also absent in the corresponding XRD patterns, suggesting the high dispersion

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Figure 4. Morphology of Cr

STEM-HAADF images of Cr

2

O

3

and Pt-Cr

2

O

3

Catalysts

.

2

O

3

(A and B) and Pt-Cr O

2 3

(C and D). See also Figure S5.

x y

of Pd and Rh species. The STEM images of Pd-Fe O material were collected

in Figures 5A, 5B, and S7, which displayed honeycomb-like matrix with abundant

pores from surface to interior. Pd NPs with an average size of 3.1 nm decorated the

surface, and the Pd loading was 6.3 wt % by ICP-AES. Besides, the well-dispersion

of Pd NPs was directly observed by the energy dispersive X-ray spectrometry

(

EDX) elemental mapping images of Pd and Fe species (See Figures 5C–5E). As to

Rh-Co catalyst, no Rh NPs were observed in the STEM images of Rh-Co

even though we have tried many different areas of copper mesh (See Figure S8).

But ICP-AES spectrum of Rh-Co showed the presence of Rh element (Rh: 7.6 wt

). Based on the results above, it seems fair to say that the NaCl-based solid-solution

3

O

4

3 4

O

3

O

4

%

functions well for dispersing noble metal species within oxide catalyst.

By IMP or DP methods, loading active components into metal oxides usually leads

to the blockage of porous channels, which in turn causes a decrease in the SSA of

the carrier. This issue could be somewhat avoided in current method. For example,

2

the SSAs of Rh-Co

3

O

4

(142 m /g) and Pd-Fe

x

O

y

(172 m /g) were higher than that

2

of Co

3

O

4

(110 m /g) and Fe

x

O

y

(100 m /g), respectively. The synthetic steps (e.g.,

milling parameters, grinding time, calcination temperature, and washing steps)

for metal oxides and noble metal loaded metal oxides were almost identical,

whereas the addition of noble metal chloride or not was different. Therefore, the

increased surface areas for supported catalysts can be ascribed to the enhanced

dispersion effect by noble metal species.

8

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x y

Figure 5. STEM Images and EDS Mapping Images of Pd-Fe O

(

A and B) are STEM images of Pd-Fe

C) STEM image of Pd-Fe

D) Fe EDX elemental mapping image for Pd-Fe

E) Pd EDX elemental mapping image for Pd-FexOy catalyst.

x y

O with scale bar at 50 and 5 nm, respectively.

x

y

O .

x

y

O catalyst.

See also Figure S7.

Normally, with the high dispersion of noble metal on the support, the interaction

5

between support and noble metal is always interesting. Therefore, a series of

H

2

-temperature programmed reduction (H

2

-TPR) spectra of Rh-Co

3

O

4

, Pt-Cr

2

O

3

,

and Pd-Fe

x

O

y

3 4

were performed (See Figure S9). As for Rh-Co O and Pt-Cr

ꢂ

two evident reduction peaks prior to 150 C were attributed to the reduction of Rh

ꢂ

species and Pt species (Rh: ꢁ125 C, Pt: ꢁ118 C), due to the strong redox capacity

ꢂ

of RhO

of the Co

and Cr

reduction temperature indicates that preferred contact or interaction between

noble metal species and Co or Cr . As for Pd-Fe , no PdO reduction

peak could be observed, possibly due to the reduction of Pd oxides before measure-

x

and PtO

and Cr

occured at ꢁ360 C

x

. The peaks at 223 C and 187 C were assigned to the reduction

3

O

4

2

O

3

, respectively, whereas the reduction of pure Co

3 4

O

ꢂ

56,57

ꢂ

58

2

O

3

and ꢁ310 C, respectively. This decreased

3

O

4

2

O

3

x y

O

5

9

ꢂ

ment temperature. The reduction peak around 320 C could be attributed to the

6

0

2 3

reduction of Fe O .

Catalytic Performance of Catalysts

3 4

The Pd-Fe , Pt-Cr , and Rh-Co O materials developed by this NaCl

x

O

y

2

O

3

solid-solution method possessed high SSAs and well-dispersed noble

metal NPs, which were supposed to show good catalytic performance. Then,

we chose three important redox processes as model reactions to evaluate their

activity.

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Figure 6. CH

4

Combustion Performance of Rh-Co

(1% in air) combustion curve over Rh-Co

B) Light-off curves of methane combustion for Rh-Co

3 4

O

ꢀ1

.

(

A) The CH

4

3

O

4

under 20,000 mL$h $g

4

in three heating-cooling cycles (1% CH in

3

O

4

ꢀ

1

ꢀ1

air under 20,000 mL$h $g ). Color line, heating part of the cycle; black line, cooling part of the

cycle.

(

C) The CH

velocities.

4 3 4

D) Arrhenius plots of the reaction rate (ln r) versus 1/T for CH combustion over Rh-Co O .

4 3 4

(1% in air) combustion curves over Rh-Co O under increased weight hourly space

(

4

CH Combustion

Emission of low concentration methane from natural gas vehicle and coal mine is

becoming an urgent issue, because of its low explosive limit in air and the strong

6

1

greenhouse effect of methane. Toward this end, catalytic combustion of methane

6

2

3 4

at low temperature is an effective strategy. Because Co O has a spinel structure,

valence states of transition-metal cations in either tetrahedral or octahedral

interstices are variable, and the surface lattice oxygen anions have different

6

3

coordination numbers, which results in different Lewis basic sites on the surface.

These unique characteristics endow Co with excellent catalytic activity in

combustion reaction. In order to improve the performance of cobalt-based catalysts,

the usual strategy is to optimize the crystal surface and SSA of Co , and supported

In fact, Rh and Pd are next to each other in the periodic

table of elements, Rh exhibits similar activity to Pd in catalytic combustion but

3

O

4

3

O

4

6

3–65

active noble metal NPs.

6

6–68

with better thermal durability.

3 4

Therefore, the porous Rh-Co O with a high

SSA value was investigated in methane combustion.

As shown in Figure 6, Rh-Co

CH . The ignition temperature T10 (at which temperature 10 % methane was

converted) of Rh-Co

3 4

O afforded a good activity in catalytic combustion of

4

ꢂ

3

O

4

was relatively low, only 260 C. Beyond T10, CH

4

conversion

ꢂ

rapidly increased, whereas the total oxidation was complete at 370 C (T99). To

10

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Figure 7. Stability Tests of Rh-Co

3 4

O

ꢂ

(

A and B) Shown in (A) and (B) are the stability tests of Rh-Co

3

O

ꢀ

4

1

under 320 C and 680 C,

ꢀ1

respectively. Reaction conditions: WHSV of (A) is 20,000 mL$g $h and WHSVs of (B) are

ꢀ ꢀ1 ꢀ1 ꢀ1

1

2

0,000 mL$g $h and 180,000 mL$g $h , respectively.

evaluate the stability, we then reused Rh-Co

3

O

4

three times under cooling-heating

ꢂ

cycles between 100 C and 450 C. The heating portions of three conversion curves

ꢂ

in Figure 6B exhibited similar T90 values, 366 C for 1st, 370 C for 2nd, and 371 C

for 3rd. Besides, among all light-off cycles, the curves of cooling and heating

part were close to coincident, implying the kinetic transformation of CH

at varied temperature.

4

was stable

3 4 4

To further evaluate the activity of Rh-Co O , we tested CH light-off curves under

various weight hourly space velocities (WHSVs). As shown in Figure 6C, the catalytic

performance was almost maintained with the increase of WHSV from

ꢀ

1

ꢀ1

ꢀ1 ꢀ1

2

0,000 mL$g $h to 40,000 and 60,000 mL$g $h , which was evident by the

ꢀ1

similar light-off curves. When continuing to increase WHSV to 120,000 mL$g $h

,

ꢂ

the CH

4

conversion begun to decrease, and the T90 shifted to 469 C. In view that the

external diffusion limitation can be overlooked under high WHSV, the calculation

of activation energy (E ) was undertaken by using the CH conversion data (CH

conversion between 5% and 15%) at 120,000 mL$g $h . The calculated Ea

a

4

ꢀ1

was 63 kJ$mol (See Figure 6D), which was comparable to Pd-based excellent

6

9,70

catalysts.

Co and other catalysts was shown in Table S1, whereas this Rh-Co

sized via mechanochemical NaCl-templated method exhibits good catalytic activity.

A more comprehensive comparison of the performance of Rh-

3

O

4

3 4

O

synthe-

CH

thermal stability is required. Although Pd-based catalysts are excellent in CH

combustion, stability has always been a difﬁcult issue because of the sintering of

4

combustion catalysts are often used in high temperature, therefore a good

4

7

1

active Pd species and the decomposition of PdO at high temperatures. In general,

the sintering problem could be modiﬁed by the introduction of metal NPs with

7

2

ꢂ

higher melting points. With a melting point of 1,955 C (metallic Rh), the thermal

stability of Rh-Co

3

O

4

was then studied in detail (Figure 7A). After continuous oper-

exhibited a fairly stable performance, there was

ꢂ

ation at 320 C for 80 h, Rh-Co

3

O

4

no signiﬁcant change in the conversion of methane. In order to further investigate

ꢂ

the potential of Rh-Co

3

O

4

, we raised the reaction temperature to 680 C and evalu-

ꢀ

1

ꢀ1

ated its high temperature stability. Under a low WHSV (20,000 mL$g $h ), Rh-

Co remained 100% CH conversion during continuous combustion for 100 h

See Figure 7B). The stability of Rh-Co was further investigated under a high

WHSV (180,000 mL$g $h ). Although the reaction condition was rather harsh

3

O

4

(

3 4

O

ꢀ

1

ꢀ1

ꢀ

1

ꢀ1

ꢂ

(

high WHSV: 180,000 mL$g $h and high temperature: 680 C), the Rh-Co O

3 4

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Table 2. Hydrogenation of Nitrobenzene under Different Conditions

ꢂ

a

Entry

Catalyst (mg)

Temp. ( C)

Solvent

ethanol

methanol

Conv. (%)

41

1

2

3

4

5

6

7

8

20

10

60

70

90

70

50

30

94

>99

84

H

2

O

80

Abbreviations are as follows: Temp, Temperature; Conv, Conversion

a

2

Reaction condition: solvent 20 mL, 5 mmol nitrobenzene, H 10 bar, and reaction time 1 h.

still could keep 75% catalytic performance after 100 h (the lighting-off curve of CH

4

ꢀ1

under a WHSV of 180,000 mL$g $h was shown in Figure S10). With high CH

combustion activity and stability, the Rh-Co

3

O

4

prepared from NaCl method is a

promising system to treat low concentration methane.

Nitrobenzene Hydrogenation

Aniline is a key intermediate for the manufacture of polyurethane, indigo, and other

industrial chemicals, whereas the selective hydrogenation of nitrobenzene is one of

7

3

the most effective methods to produce aniline. Pd catalysts supported on carbon,

MCM-48 and mesoporous metal oxides (TiO , ZrO , and Al ) have been exten-

2

2 3

O

sively studied with excellent hydrogenation performance, however, recycling the

used palladium-containing catalysts is quite complex in liquid-phase hydrogena-

7

4–77

tion.

because it is easy to be recycled, and then the current Pd-Fe

Magnetic separable material is an attractive heterogeneous catalyst

x

O

y

catalyst with mag-

netic feature, high surface area, and small Pd NPs was applied to the hydrogenation

of nitrobenzene.

A preliminary attempt to optimize the hydrogenation process at different tempera-

tures was carried out with ethanol as solvent. As the reaction temperature increased

ꢂ

from 60 C to 90 C, a rapid increase of nitrobenzene conversion from 44% to >99%

occurred (Table 2). Aniline was the sole found product, whereas side products such

as nitrosobenzene, N-phenyl hydroxylamine, azoxybenzene, azobenzene, and hy-

drazobenzene cannot be detected on GC. To further improve the performance of

Pd-Fe

x

O

y

, the effect of solvent in nitrobenzene hydrogenation was investigated.

ꢂ

When the reaction temperature was ﬁxed at 70 C, the conversions of nitrobenzene

2

were 98% in methanol solvent and 100% in water. With H O as the green solvent, we

continued to optimize the reaction temperature. When the temperature was

ꢂ

reduced to nearly room temperature (30 C), the Pd-Fe

x

O

y

still exhibited good

performance with an 85% conversion of nitrobenzene. Even when the amount of

Pd-Fe catalyst is halved, an 80% conversion of nitrobenzene still was preserved.

x

O

y

x y

To study the scope of this Pd-Fe O catalyst, various derivatives of nitrobenzene

such as o-Nitrotoluene, 4-nitrochlorobenzene, 4-nitroanisole, 1-Bromo-4-Nitroben-

ꢂ

zene, and p-Nitrotoluene were hydrogenated at 50 C. As shown in Table 3, all ﬁve

derivatives with either electron-withdrawing or electron-donating groups were

successfully hydrogenated to corresponding amines with high selectivity. Especially,

the catalytic hydrogenation of o-Nitrotoluene, a molecule with steric hindrance,

x y

could be ﬁnished in 50 min, revealing the general activity of Pd-Fe O catalyst.

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Table 3. Hydrogenation of Nitrobenzene Derivatives under Different Conditions

a

b

Entry

Substrate

Product

Time (min)

Conv.

Sel.

1

50

>99%

2

3

20

>99%

4

5

40

30

>99%

Reaction condition: solvent 20 mL, 5 mmol nitrobenzene derivative, and 20 mg Pd-Fe

x

O

y

. Temperature

ꢂ

5

0 C, H

2

10 bar, and reaction time 1h.

a

Conv, conversion.

b

Sel, selectivity.

To study the catalyst stability, spent Pd-Fe

x

O

y

solid was separated by the action of

catalyst was rather

x y

external magnet, which was reused for the next run. The Pd-Fe O

stable in the ﬁve cycles of hydrogenation reaction, during which the nitrobenzene

conversions were kept around 68% (See Figure 8A). The dispersion of Pd NPs on

the reused Pd-Fe

energy loss spectroscopy (EELS) (See Figures 8B and S11). Pd NPs were well-

dispersed on the Fe support. The interaction between Pd NPs and Fe might

during nitrobenzene

x y

O catalyst was explored by STEM-EDX mapping and electron

x

O

y

x y

O

have contributed to the reaction stability of Pd-Fe

hydrogenation.

x y

O

x y

Figure 8. Stability Measurement of Pd-Fe O Catalyst in Nitrobenzene Hydrogenation

(

A) Reusing runs of Pd-Fe

x

O

y

. Reaction condition: 20 mL H

2

O, 20 mg Pd-Fe

10 bar.

x y

O (recycled), reaction

ꢂ

time 1 h, 15 mmol nitrobenzene, temperature at 50 C, and H

2

(

x y

B) STEM and EDX mapping images of the reused Pd-Fe O .

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Figure 9. Conversion of

Nitrobenzene versus Time in Scale-

up Experiment

Reaction conditions: 30 mL

x y

nitrobenzene, 20 mg Pd-Fe O ,

ꢂ

temperature at 80 C, and H

bar.

2

20

With the good activity, selectivity and stability of Pd-Fe

was further tested in a scale-up experiment, during which 30 mL nitrobenzene

without solvents was hydrogenated by 20 mg Pd-Fe catalyst. The plot of

x y

O on hand, this catalyst

x

O

y

nitrobenzene conversion versus time was shown in Figure 9. Aniline was the main

product under solvent-free condition with a high selectivity above 98% (by gas

chromatography, GC). The conversion of nitrobenzene was approximately propor-

tional to reaction time, indicting the reaction might follow zero-order kinetics

with respect to nitrobenzene under ﬁxed H

2

pressure, which was consistent with

7

8

previous reports. Meanwhile, the Pd-Fe

x

O

y

catalyst offered a high TON value

4

(

mol nitrobenzene converted per mol Pt) of 2.49 3 10 .

After the reaction was over, the aniline could be simply separated and collected

3

by centrifugation, due to the different density of aniline (1.02 g/cm ) and water

3

(

1.00 g/cm ), and comparisons of samples before and after centrifugation are

shown in Figure S12. The isolated yield of aniline was 90% and the product was

1

characterized by proton nuclear magnetic resonance ( H NMR) (See Figure S13).

By combination of magnetic feature for recycling, high activity and selectivity, and

x y

good performance in scale-up process, the Pd-Fe O catalysts might provide an

attractive material for nitrobenzene hydrogenation.

RWGS Reaction

7

9–81

In recent years, carbon dioxide utilization is one of the hottest scientiﬁc topics.

The RWGS for carbon dioxide hydrogenation to carbon monoxide is a promising

8

2

catalytic process. Through Fischer-Tropsch synthesis, the obtained CO can be

further converted into a variety of liquid fuels (gasoline, diesel, and methanol) and

7

6,83

high value oxygenates.

In general, catalysts which offer excellent reaction

rates in forward reaction also show high reaction rate in reverse reaction. Although

Pt-based catalysts were widely used in WGS reaction, there is still large room to

optimize the catalytic activity in RWGS reaction. On the other hand, the highly

2

porous Cr

2

O

3

with a SSA of 224 m /g might serve as a potential oxide support, which

would promote dispersion of Pt. Therefore, the Pt-Cr

in RWGS.

2 3

O material was then studied

2 3

The catalytic performance of Pt-Cr O in RWGS was exhibited in Figure 9. When

ꢀ1

2

WHSV was ﬁxed at 20,000 mL$g $h , the conversion of CO to CO begun at a

ꢂ

temperature as low as 200 C. With the increase of temperature, the conversion of

ꢂ

CO

2

was enhanced. When the reaction temperature arrived at 500 C, the conversion

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Figure 10. CO

2

Conversion and CO Selectivity of Pt-Cr

2

O

3

Catalyst in the RWGS

:H =1:3; and WHSV = 20,000;

ꢂ

Reaction conditions: atmospheric pressure, 200 C–500 C; CO

2

ꢀ1

6

0,000; and 80,000 mL$g $h .

of carbon dioxide reached 44%. Besides, the selectivity of CO can keep at more

ꢂ

than 98%, when the temperature exceeded 300 C. Moreover, the catalytic perfor-

2 3

mance of Pt-Cr O could be maintained at an increased space velocity. Under the

ꢀ1

2

conditions of WHSV = 40,000 and 80,000 mL$g $h , 40% and 35% CO conver-

ꢂ

sion still could be obtained at 500 C (see Figure 10).

8

4

Actually, the stability of Cu-based catalysts was often unsatisfying in RWGS, so

2 3 2

we then moved to study the stability of Pt-Cr O in CO hydrogenation. Even at

ꢂ

5

00 C for running for 100 h, both CO

2

conversion and CO selectivity could be

maintained (see Figure 11). More importantly, compared with fresh catalyst (average

particle size of Pt: 3.2 nm), no evident sintering of Pt NPs was observed after the

ꢂ

1

00 h CO

average particle size of reused Pt-Cr

porous and dispersed Pt-Cr synthesized by NaCl-based solid solution has

2

hydrogenation under a high temperature of 500 C (Figure 11B). The

2

3

O catalyst was 4.9 nm. Hence, the highly

2

O

3

great potential in a RWGS reaction.

Conclusion

In summary, we have developed a one-pot protocol to fabricate porous metal

oxides with ﬁne dispersed noble metal NPs via a NaCl-based solid-solution

approach. It was found that the well-dispersion of metal chlorides on NaCl, that is

the mechanochemical formation of MClx-NaCl, solid solution was a key step to con-

trol the porosity of metal oxides and the dispersion of noble metal species. In this

regard, a series of porous metal oxides and related catalysts (Fe

x y 2 3

O , Cr O ,

2

Co , Pd-Fe , Pt-Cr , and Rh-Co ) with a high SSA up to 224 m /g can

3

O

4

x

O

y

2

O

3

3 4

O

successfully be prepared. By combination of high SSA and highly dispersed noble

metal centers, catalysts obtained by this new route afforded excellent activity and

4

stability in three key redox reactions, such as: CH combustion, hydrogenation of

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2 3

Figure 11. Stability Test of Pt-Cr O

(

A) The performance of Pt-Cr

2

O

3

catalyst in the RWGS reaction within 100 h. Reaction conditions:

ꢂ

ꢀ1 ꢀ1

atmospheric pressure, 500 C; CO

2

:H

2

ratio = 1:3; and WHSV = 20,000 mL$g $h

.

(

B) The STEM image of Pt-Cr

2

O

3

after stability test.

nitrobenzene, and RWGS reaction. A summary comparing the porosities and cata-

lytic activities of the three catalysts with other methods suggests the efﬁciency of

this NaCl-based mechanochemical method (see Table S2). All in all, this simple,

effective, and solid state synthesis technique might offer a new way for engineering

metal oxide-based catalysts, especially toward large-scale applications.

EXPERIMENTAL PROCEDURES

Resource Availability

Lead Contact

Further information and requests for resources and reagents should be directed to

the Lead Contact, Pengfei Zhang (chemistryzpf@sjtu.edu.cn ).

Materials Availability

This study did not generate new unique reagents.

Data and Code Availability

This study did not generate datasets or codes.

Full experimental procedures are provided in the Supplemental Information.

SUPPLEMENTAL INFORMATION

Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.

020.04.003.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China

(

21776174), the Thousand Talent Program, the Open Foundation of the State Key

Laboratory of Ocean Engineering (Shanghai Jiao Tong University of China) (1809),

Shanghai Jiao Tong University Scientiﬁc and Technological Innovation Funds

(

2019QYB06), China Shipbuilding Industry Corporation (CSIC), and Zhejiang Xinan

Chemical Industrial Group. H.C., Z.B., Z.W., and S.D. were supported by the U.S.

Department of Energy, Ofﬁce of Science, Ofﬁce of Basic Energy Sciences, Chemical

Sciences, geosciences, and Biosciences Division, Catalysis Science program.

AUTHOR CONTRIBUTIONS

P.Z. and S.D. designed the research idea and experiments. Y.S., N.C., X.D., H.C.,

Z.B., and Z.W. together completed related experiments, and S.Y. was responsible

for the STEM characterization. P.Z. and Y.S. wrote the paper.

16

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. Wang, Y., Arandiyan, H., Scott, J., Bagheri, A.,

ll

Article

DECLARATION OF INTERESTS

The authors declare no competing interests.

Received: November 5, 2019

Revised: January 10, 2020

Accepted: April 2, 2020

Published: April 29, 2020

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