Co3O4 nanorods with great amount oxygen vacancies for highly

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Co3O4 nanorods with great amount oxygen vacancies for highly efficient Hg0 oxidation Xiaopeng Zhang, Hang Zhang, Hongda Zhu, Chengfeng Li, Ning Zhang, Junjiang Bao, and Gaohong He Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00765 • Publication Date (Web): 02 Jun 2019 Downloaded from http://pubs.acs.org on June 6, 2019

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The removal of Hg0 on Co3O4 exposing (2 2 0) facet

ACS Paragon Plus Environment

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Co3O4 nanorods with a great amount of oxygen vacancies for highly efficient Hg0

2

oxidation from coal combustion flue gas

3

Xiaopeng Zhang, Hang Zhang, Hongda Zhu, Chengfeng Li, Ning Zhang, Junjiang Bao*, Gaohong

4

He*

5

School of Petroleum and Chemical Engineering, State Key Laboratory of Fine Chemicals, Dalian

6

University of Technology, Panjin, 124221, China

7

Abstract

8

Oxidizing elemental mercury (Hg0) to Hg2+ is an effective way to remove Hg0 from flue gas.

9

Surface active oxygen species are considered to be important active sites in Hg0 oxidation process.

10

The concentration enhancement of surface active oxygen species is a primary challenge for this

11

technology. Oxygen vacancies can easily capture and activate gaseous oxygen forming more

12

surface active oxygen species, which may lead to a better Hg0 oxidation efficiency. Co3+ in Co3O4

13

can generate oxygen vacancies through the reduction of Co3+ to Co2+ and the oxygen vacancies

14

formation process is controlled by Co2+/Co3+ ratio. Inspired by this, Co3O4 nanorods exposing

15

(220) facet with a high Co3+/Co2+ ratio were successfully synthetized. Raman and XPS results

16

show that the high concentration Co3+ leading to more oxygen vacancies. It results in a better

17

catalytic performance for Co3O4 nanorods whose Hg0 oxidation efficiency maintains above 90% at

18

180,000 h- in the temperature range of 100-300 °C. After 2880 min reaction, the Hg0 oxidation

19

efficiency of Co3O4-nanorods reduces to about 72% and it recovers to the original level after in

20

situ thermal treatment at 550 ºC suggesting a great renewable property. Furthermore, XPS results Corresponding author: Junjiang Bao. E-mail: [email protected]

Tel: +86427-2631803

Corresponding author: Gaohong He. E-mail: [email protected]

Tel: +86427-2631916 1


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of Co3O4-nanorods before and after reaction show that the concentration of Co3+ and surface

2

active oxygen decrease after reaction. A reaction mechanism was revealed based on these results.

3

Hg0 reacts with surface active oxygen forming HgO and the consumed oxygen is replenished by

4

gaseous O2. Co3+/Co2+ redox couple can improve the electron transfer activity to enhance the Hg0

5

oxidation efficiency in the presence of O2. The effects of flue gas components on Hg0 oxidation

6

efficiency are also investigated. O2 and NO have positive effects while H2O and SO2 have

7

negative effects on Hg0 removal process. But the Co3O4 nanorods still have an efficiency of 75%

8

even in the presence of 8% H2O and 200 ppm SO2.

9

Key words:

10

Elemental mercury; Oxidation; Oxygen vacancies; Co3O4; Crystal facet

11

1. Introduction

12

Mercury as a major pollutant in coal-fired flue gas has got worldwide attention due to its

13

extreme toxicity, high volatility, persistence and bioaccumulation in the environment 1-3. Mercury

14

is present in coal combustion flue gas in three different forms, particle-bound mercury (Hgp),

15

oxidized mercury (Hg2+) and elemental mercury (Hg0)

16

difficult to remove through traditional environmental protection equipment because of its high

17

volatility and insolubility 6. It has been proven that converting Hg0 to an easily removed form of

18

Hg2+ is a feasible way to control Hg0 emission from flue gas 7, 8.

19

Various transition metal oxides, such as VOx

9, 10,

4, 5.

WOx

Among these forms, Hg0 is the most

11, 12,

MnOx

13, 14,

ZrOx

15,

RuO2

16,

20

CoOx

21

favorable features of low cost, earth abundance and good stability has been considered to be a

22

promising catalyst

17, 18

and CeOx 19, 20, have been widely used as catalysts for Hg0 oxidation. CoOx with the

21.

Co3O4 with a unique redox couple Co2+/Co3+ is one of the most efficient 2


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1

active constituents for Hg0 oxidation

2

can lead to a good redox property. Mei et al.

3

showed a relatively good Hg0 removal efficiency of 70% at high temperature. In recent years,

4

some researches point that surface chemisorbed oxygen as an important active site in Hg0 catalytic

5

oxidation reaction can greatly facilitate the Hg0 oxidation process by Hg0 (ad) + O* (surface active

6

oxygen) → HgO. Hence increasing the amount of surface active oxygen will further improve the

7

catalytic efficiency for Hg0 effectively. Metal iron doping is a general method to increase the

8

amounts of surface active oxygen 24. Zhang et al. reported that Ce-doped Co3O4 could obtain 50%

9

more surface active oxygen when compare to single Co3O4

22

due to that the active electron transfer between Co2+/Co3+ 23

synthesized spinel Co3O4 for Hg0 removal, and it

18

resulting in a 30% higher Hg0

10

oxidation efficiency below 150 oC. In the meanwhile, decreasing catalyst particle size can increase

11

the number of low-coordination atoms located at the edges and corners

12

and active more gaseous oxygen leading to a better Hg0 oxidation performance. Esswein et al.

13

researched Co3O4 nanoparticles with different particle size contrastively and found the small-sized

14

Co3O4 exhibits better performance

15

controlled accurately by doping metal iron or decreasing the particle size, and the surface active

16

oxygen is need to be further enhanced 28, 29.

17

27.

25, 26

which will capture

However, the catalysts synthetic process is unable to be

Oxygen vacancies as one type of special microstructures can easily adsorb gaseous oxygen and

18

form more surface active oxygen

19

reduction of Co3+ to Co2+. Therefore, the proper control of Co2+/Co3+ ratio can control the

20

formation of oxygen vacancies

21

Co3O4, synthesizing and regulating Co3O4 nanocatalysts exposed different lattice plane is an

22

attractive method to control Co3+ amounts on catalysts surface, which will then control the

30.

29, 31.

Co3+ in Co3O4 can generate oxygen vacancies through the

As the ion valence is different in different lattice plane of

3


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formation of oxygen vacancies and the Hg0 oxidation ability of catalysts. It has been reported by

2

Xie et al.

3

exposure of the {110} plane will lead to a higher Co3+/Co2+ ratio, which may give more oxygen

4

vacancies resulting in a good Hg0 oxidation efficiency.

32

the Co3+ cations are only present on the {110} plane of Co3O4. Thus, the greater

5

In the present work, Co3O4 catalysts exposing more {110} plane were synthesized for Hg0

6

oxidation and to make a contrastive investigation with general Co3O4 nanoparticles. BET, XRD,

7

TEM and XPS analysis were performed to characterize physicochemical properties of the

8

catalysts.

9

2. Experimental

10

2.1 Preparation of catalysts

11

Co3O4 nanorods (denoted as Co3O4-nanorods) were prepared by the method of ethylene glycol

12

precipitation. 4.98 g cobalt acetate tetrahydrate was dissolved in 60 mL ethylene glycol and the

13

obtained solution was heated to 160 oC under vigorous stirring and a continuous flow of nitrogen.

14

Then 200 mL aqueous Na2CO3 solution (0.2 mol/L) was added dropwise into the cobalt acetate

15

solution. The mixtures of liquid and sediments were stirred for 1 h under the N2-flow. Blue

16

powders were obtained by vacuum filtration and washing. Finally, the obtained products were

17

dried at 60 °C overnight and calcined at 450 °C for 4 h.

18

Co3O4 nanoparticles (denoted as Co3O4-nanoparticles) were synthesized by the same method

19

and same reactant except for the N2-flow. The whole process of synthesizing Co3O4 nanoparticles

20

are carried out in the air atmosphere.

21

2.2 Characterization of catalysts

22

The crystalline phases and crystallinity of the catalysts were measured by the powder X-ray 4


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1

diffraction (XRD) with XRD-7000 S system using Cu Kα radiation (40 kV, 100 mA)

2

(SHIMADZU Corporation, Japan). Scanning velocity is 5° min-1 with a step size of 0.02°.

3

The TEM and HR-TEM were performed on Hitachi HT7700 Transmission electron

4

microscopic (TEM) operating at 100 kV to investigate the particle sizes, morphologies and lattice

5

plane of the catalysts. The samples were ultrasonically dispersed in ethanol. Then prepare the

6

specimen by dropping it onto a clean holy copper grid and drying it in air.

7

The N2 adsorption was characterized at -196 °C on an Autosorb-iQ-C automated gas sorption

8

system (Quantachrome Instruments, USA). The specific surface areas of the catalysts were

9

calculated by multipoint BET analysis of the N2 adsorption isotherm. All the samples were

10 11 12

degassed at 300 °C for 2 h prior to the measurements. The Raman spectra were recorded on a Renishaw inVia spectrometer with a 532 nm emission line.

13

The ion valence and atomic concentration of metal species on catalyst surface were measured

14

by x-ray photoelectron spectroscopy (XPS) using an ESCALAB250 (Thermo Fisher Scientific

15

Corporation, USA) with monochromatic Al Kα radiation. The charging effects of measurement

16

were eliminated by correcting the observed spectra with the C 1s binding energy value of 284.6

17

eV.

18

Hydrogen temperature-programmed reduction (H2-TPR) was performed to analyze the redox

19

properties on an Autosorb-iQ-C automated gas sorption system (Quantachrome Instruments,

20

USA). Before the measurement, 50 mg samples were treated in He atmosphere at 300 °C for 2 h

21

to clean the surface. Then cool the samples down to 50 °C in the same atmosphere. Subsequently,

22

the samples were heated and temperature-programmed reduction under 10 vol% H2/Ar (30 5


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mLmin-1), the heating rate is 10 °C min-1 and the heating rate is 50-900 °C. The H2 consumption

2

amount was quantitatively measured by a thermal conductivity detector (TCD).

3

2.3 Catalytic activity measurement

4

Hg0 removal efficiency was measured in a fixed-bed flow reactor (Fig. 1). A mercury

5

permeation tube (VICI, Metronics Inc.) as a Hg0 source was placed in a U-shaped glass tube,

6

which was immersed in a constant temperature (40 °C) water bath. A simulated flue gas consisting

7

50 μg/m3 Hg0, 5 vol% O2, NO (when used), H2O (when used) and SO2 (when used) and balance

8

N2 was used. NO and SO2 were controlled by mass flow controllers and water vapor was

9

generated by a heated water bubbler. The total gas flux was 600 mL/min. Catalyst usage amount is

10

0.2 mL (about 0.22 g) and the gas hourly space velocity (GHSV) is about 180,000 h-1. All the feed

11

gases were mixed and preheated in a gas mixing chamber before entering the reactor. The balance

12

N2 was divided into two branches: one branch converged with the individual streams of O2 formed

13

the main gas flow, and the other branch (200 mL/min) passed through the U-tube to introduce Hg0

14

vapor into the reactor system. To avoid mercury condensation, silicone pipelines were warmed at

15

100 °C by heating belts. An online mercury analyser (VM-3000, Mercury Instruments Analytical

16

Technologies, Germany) was employed to measure Hg0 concentrations of the inlet and outlet of

17

the reactor (denoted as Hg0in and Hg0out). To eliminate the interference of Hg0 adsorption, the

18

catalysts reached adsorption equilibrium in Hg0 balance N2 atmosphere. The Hg0 oxidation

19

efficiency (Eoxi) can be defined as follow:

20

Eoxi 

0 Hgin0  Hg out 100% Hgin0

6


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1 2

Fig. 1 Schematic diagram of the fix-bed experiment system

3

3. Results and discussion

4

3.1 Catalysts characterization

5

3.1.1 TEM and HR-TEM

6

TEM and HR-TEM measurements were performed to investigate the microstructures and

7

exposed crystal facet of the two catalysts. As shown in Fig. 2A and B, the morphology of the two

8

catalysts is definitely confirmed as nanorod and nanoparticle respectively, and their grain sizes are

9

in the same order of magnitude. The lattice fringes of Co3O4-nanorods and Co3O4-nanoparticles can

10

be noticed in the HR-TEM images (shown in Fig. 2A’ and B’), and the crystalline interplanar

11

spacing that measured and calculated by Fast Fourier Transform (FFT) of HR-TEM images are

12

marked in those two images. According to the crystalline interplanar spacing in Fig. 2A’ and B’,

13

the most lattice fringes of Co3O4-nanorods can be ascribed to the (220) plane (belongs to {110}

14

plane) and lattice fringes of Co3O4-nanoparticles mainly belong to the (111) and (311) plane. This

15

phenomenon proves that two different Co3O4 structures of nanorods and nanoparticles were

16

successfully synthetized. As can be seen in Fig. 2 A’’ and B’’, Co3O4-nanoparticles have clear 7


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lattice fringes while some parts of lattice images became blurred for Co3O4-nanorods which

2

indicates that Co3O4-nanorods have more oxygen vacancies 33, 34.

3 4

Fig. 2 (A) TEM and (A') (A’’) HR-TEM of Co3O4-nanorods, (B) TEM and (B') (B’’) HR-TEM of

5

Co3O4-nanoparticles

6

3.1.2 XRD

7

The XRD patterns of Co3O4-nanorods and Co3O4-nanoparticles are shown in Fig. 3. Obvious

8

diffraction peaks of Co3O4 can be detected in both of the two catalysts. The peaks at 2θ=31.27°,

9

36.85°, 44.81°, 59.35° and 65.23° are respectively indexed to the (220), (311), (400), (511) and

10

(440) crystal planes of Co3O4 (PDF# 43-1003). Scherrer particle sizes corresponding to different

11

facets are the sample sizes perpendicular to these facets. It gives a direct information of the growth

12

orientation and the vertical facet exposed degree of the samples. As shown in Table 1, Scherrer

13

particle sizes corresponding to (220) and (440) facets of Co3O4-nanorods are higher than those of

14

Co3O4-nanoparticles, but Scherrer particle sizes corresponding to the other three facets of 8


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Co3O4-nanorods are lower than those of Co3O4-nanoparticles. It suggests that Co3O4-nanorods

2

grow in the direction vertical to (220) plane and expose more (220) plane. These results are

3

consistent with the results of TEM and HR-TEM.

Co3O4-nanorods

（440（

（511（

（400（

（220（

（311（

1

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Co3O4-nanoparticles

30

4 5 6 7

40

2 ()

50

60

70

Fig. 3 XRD patterns of the catalysts Table 1 The Scherrer particle sizes of Co3O4-nanorods and Co3O4-nanoparticles Crystalline grain sizes of the diffraction peaks (nm) Catalysts

8

(220)

(311)

(400)

(511)

(440)

Co3O4-nanorods

18.283

13.992

15.831

98.097

33.970

Co3O4-nanoparticles

16.660

17.406

19.326

104.228

32.105

3.1.3 N2 adsorption and desorption

9

The specific surface areas of catalysts were determined by N2 adsorption and desorption. As

10

shown in Table 2, Co3O4-nanorods have a much larger surface area than Co3O4-nanoparticles.

11

Larger surface area is usually corresponding to more available surface active sites which will

12

adsorb more reaction components resulting in a higher catalytic activity of heterogeneous catalysis

13

35.

14

Table 2 Structural parameters of catalysts measured by N2 adsorption Catalysts Co3O4-nanorods

Surface area (m2/g) 115.80 9


Pore volume (cc/g) 0.678

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Co3O4-nanoparticles

1

70.01

0.411

3.1.4 Raman A1g

Eg

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

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F 2g

Co3O4-nanorods

2

F 2g A1g

Eg

200

1

F 2g

400

Co3O4-nanoparticles

2

F 2g

600

800 -1

1000

1200

2

Raman shift (cm )

3

Fig. 4 Raman spectra of the catalysts

4

Raman patterns of the two catalysts are show in Fig. 4. Four peaks around 475, 517, 614 and

5

680 cm-1 are observed in both of the two catalysts, which are corresponding to Eg, F12g, F22g and

6

A1g modes of the Co3O4 spinel structure 36, 37. As has been reported, the peak at 687 cm−1 is due to

7

the sublattice with highest valency cations

8

lattice distortion

9

broadened when compared to Co3O4-nanoparticles (FWHM is 17.98), which is ascribed to the

10

lattice distortion 39. These results suggest that Co3O4-nanorods exposing (220) facet contains more

11

O vacancies. Xie et al.32 has pointed that Co3+ cations are only present on the {110} plane of

12

Co3O4. It suggests that Co3O4-nanorods exposing (220) have higher surface Co3+ concentration (it

13

will be confirmed by XPS results in 3.1.5) which can give more O vacancies through reduction of

14

Co3+ to Co2+ during synthetic process.

15

3.1.5 XPS

38.

36

and any differences of this peak can be ascribed to

The Raman peak of Co3O4-nanorods at 687 cm−1 (FWHM is 19.07) are

16

XPS was carried out to analyze the concentration and chemical state of surface elements. The

17

XPS patterns of Co 2p and O 1s are shown in Fig. 5 and the surface atomic concentrations are 10


Energy & Fuels

1

listed in Table 3. As shown in Fig. 5 (A), the two main peaks in Co 2p spectra can be split into

2

four overlapping peaks. The two peaks around 780 eV and 795 eV are attributed to Co3+, while the

3

other two peaks around 781 eV and 796 eV belong to Co2+

4

529.8 eV and 531.5 eV, which are ascribed to lattice oxygen (Oβ) and chemisorbed oxygen (Oα)

5

respectively

6

intense peak at 532.8 eV shows up, which is attributed to the high binding energy peak from

7

surface oxygen defect species (Oγ)

8

vacancies. These results are in great agreement with Raman results.

42, 43,

40, 41.

In Fig. 5 (B), two peaks around

can be detected in both of the two catalysts. But for Co3O4-nanorods, a new

44, 45.

It suggests that Co3O4-nanorods has more oxygen

9

As shown in Table 3, the Co3+/Co2+ ratio of Co3O4-nanorods is much higher than that of

10

Co3O4-nanoparticles, which will generate more anionic defects to adsorb and activate oxygen in

11

gas phase forming surface active oxygen. This deduction can be confirmed by the concentration of

12

Oα and Oγ which were much higher on Co3O4-nanorods than those on Co3O4-nanoparticles.

13

Chemisorbed oxygen as long as oxygen defect species were generally considered as important

14

active sites for the oxidation of Hg0

15

on Co3O4-nanorods are the key factor for the higher Hg0 oxidation activity.

46, 47,

A

thus, the higher chemisorbed Oα and Oγ concentrations

Co 2p 2+

Co

Co

Co

B O

2+

Co

800

795

790

785

Binding Energy(eV)

Intensity(a.u.)

3+

Co

3+

Co

O O

Co3O4-nanoparticles

16 17

O

Co3O4-nanorods

2+

Co

O 1s

O

3+

Co

2+

3+

Co3O4-nanorods

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Co3O4-nanoparticles

780

536

534

532

Binding Energy(eV)

Fig. 5 XPS spectra of Co 2p and O 1s for the catalysts

18 19

Table 3 Surface atomic concentration and the ratios of different chemical states 11


530

528

Page 13 of 29

Co3+/Co2+

Catalysts

1

(Oα+Oγ)/Oβ

Co3O4-nanorods

0.90

3.58

Co3O4-nanoparticles

0.63

0.57

3.1.6 H2-TPR

2

H2-TPR was performed to study the redox ability of Co3O4-nanorods and Co3O4-nanoparticles

3

and the reduction profiles are shown in Fig. 6. Both of the two catalysts have two main reduction

4

peaks due to the two steps reduction of Co3O4. The temperatures of Co3O4-nanorods reduction

5

peaks are around 297 ºC and 549 ºC, and those of Co3O4-nanoparticles are around 312 ºC and 436

6

ºC. The peak at lower temperature is attributed to the reduction of Co3O4 to CoO, while another

7

one at higher temperature is produced by the reduction of CoO to Co

8

Co3O4-nanoparticles, Co3O4-nanorods have a lower starting reduction temperature (188 ºC). In

9

addition, the reduction peak at low temperature of Co3O4-nanorods shifts to lower temperature. It

10

can be due to the surface oxygen vacancies which promote Co3+/Co2+ redox and oxygen mobility

11

leading to a higher reducibility of the catalyst 48.

48, 49.

549

297

Co3O4-nanorods 188

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

436 312

213 Co3O4-nanoparticles

100

12 13 14

200

300

400

500

Temperature (℃)

600

Fig. 6 H2-TPR profiles of the catalysts

3.2 Catalytic performance

12


700

800

Compared with

Energy & Fuels

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3.2.1 Hg0 oxidation efficiency at different temperature

2

Fig. 7 A presents the Hg0 oxidation efficiency of Co3O4-nanorods and Co3O4-nanoparticles. It

3

can be noted that Co3O4-nanorods exhibit a much higher catalytic activity than

4

Co3O4-nanoparticles. In the temperature range of 100-300 ºC, the Hg0 oxidation efficiency of

5

Co3O4-nanoparticles is below 30% and that of Co3O4-nanorods is higher than 90% with a

6

maximum of 98% at 200 °C. To investigate the morphology effects on Hg0 oxidation efficiency,

7

Co3O4-nanosheets and Co3O4-nanocubes were synthesized to make a comparison with

8

Co3O4-nanorods. TEM and HR-TEM of these two catalysts are shown in Fig. 1s. It can be seen

9

that Co3O4-nanosheets and Co3O4-nanocubes are successfully synthesized and both of the two

10

catalysts mainly expose (220) facet. Fig. 7 B shows the Hg0 oxidation efficiencies of the three

11

catalysts with different morphology. All of the three catalysts have similar Hg0 oxidation

12

efficiencies. This phenomenon reveals that the exposing facet but not the morphology of the

13

catalyst plays a key role in Hg0 oxidation process. Hg0 catalytic removal performance of common

14

Mn/Ce/Ti and W/V/Ti catalysts from 21 references were summarized to make a comparison with

15

Co3O4-nanorods and the results are shown in Table 1s. Comprehensive consideration of GHSV

16

and reaction temperature window, it can be found that Co3O4-nanorods has a higher Hg0 removal

17

efficiency at a higher GHSV in a wider temperature range. 100

100

A Hg Oxidation Efficiency ()

80

Co3O4-nanorods Co3O4-nanoparticles

60

40

20

0

18

B

80

Co3O4 -nanorods Co3O4 -nanocubes Co3O4 -nanosheets

60

40

0

0

Hg oxidation efficiency(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 29

20

0 100

150

200

o

250

300

100

Temperature( C)

150

200

Temprature (C)

13


250

300

Page 15 of 29

1

Fig. 7 Hg0 oxidation efficiency of the catalysts with different morphology

2

The catalytic performance is relative with the physicochemical properties of the catalysts.

3

Characterization results have shown that Co3O4-nanorods mainly expose (220) facet with a high

4

Co3+/Co2+ ratio, which results in more oxygen vacancies. The oxygen vacancies can easily capture

5

and activate gas phase oxygen forming active oxygen species and it plays an important role in the

6

Hg0 oxidation process. Furthermore, Co3O4-nanorods have a larger BET surface area leading to

7

more available surface active sites.

8

3.2.2 The effects of flue gas components on Hg0 oxidation efficiency 100

O2

NO

H2O

SO2

80

60

40

0

Hg Oxidation Efficiency ()

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

20

0

5%

9

O2

2 2 2 SO SO SO m m p p p p 0 0 0 0 + 0 + + 0 + 20 10 + 10 20 O2 O2 O2 O2 + + 2 + + O 2 5% 5% O2 O2 2 5% 5% 5% % O % O 5% 5% 5 5

8%

O2

m pp

O

O

N

m pp

N

O

m pp

N

0%

O H2

6%

O H2

8%

O H2

m pp

10

Fig. 8 Effects of individual flue gas components on Hg0 oxidation efficiency over Co3O4-nanorods

11

Flue gas components usually play an important role in Hg0 oxidation process. Therefore, the

12

effects of individual flue gas components on Hg0 oxidation efficiency over Co3O4-nanorods were

13

investigated and the results are shown in Fig. 8.

14

O2 shows a promotive effect on the Hg0 oxidation efficiency. It has been pointed that gaseous

15

O2 can adsorb on catalyst surface forming active oxygen species (O*). O* will react with Hg0

16

through O* + Hg0 → HgO 50. The higher O2 concentration results in more surface active oxygen 14


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

species which accelerates the reaction between O* and Hg0.

2

NO slightly enhances Hg0 oxidation efficiency. After 100 and 200 ppm NO addition, Hg0

3

oxidation efficiency increases from 92.4% to 95.2% and 96.2%, respectively. According to

4

previous works, NO can react with O2 forming NO2 with a certain oxidation ability which can

5

improve Hg0 oxidation process through NO2 + O* + Hg0 → HgNO3 51, 52.

6

H2O has prohibitive effect on Hg0 oxidation process which is mainly due to the strong

7

adsorption competition between H2O and Hg0. The prohibitive effect of H2O can be eliminated

8

after H2O cut off 46, 47.

9

SO2 has serious inhibited effects on Hg0 oxidation process. When 100 ppm SO2 is added into

10

flue gas, Hg0 oxidation efficiency declines from 92.4% to 83.2% and it furtherly declines from

11

83.2% to 75.2% after 200 ppm SO2 addition. The inhibited effects of SO2 are mainly owing to the

12

SO2 poisoning catalyst surface and the competition between SO2 and Hg0. To better understand

13

SO2 inhibited effects, SO2 is introduced into the flue gas without O2 and then into the flue gas with

14

O2. This experiment can give the detail of SO2 inhibited effects on Hg0 adsorption process and

15

Hg0 oxidation process and the results are shown in Fig. 9. When the flue gas is without O2, 200

16

ppm SO2 can deactivate the catalyst in about 20 min. It is due to that SO2 can react with Co oxides

17

forming Co sulfates and it will cut off the reaction path way of O* + Hg0 → HgO leading to total

18

loss of Hg0 adsorption capacity of Co3O4

19

increases to 85% in 20 min. SO2 can give another reaction path way in the presence of O2. SO2

20

firstly react with O* forming SO3 and then SO3 as a new adsorption site can absorb O* and then

21

react with Hg0 forming HgSO4 54, 55.

22

53.

After O2 is added, Hg0 removal efficiency sharply

SO2 + O*→ SO3 15


Page 16 of 29

Page 17 of 29

1

SO3 + O* + Hg0 → HgSO4

2

It should be noted that Hg0 oxidation efficiency (the removal efficiency after O2 addition of 85%)

3

in Fig. 9 is more than that (75%) in Fig. 8. In Fig. 8, to eliminate the effect of Hg0 adsorption, Hg0

4

oxidation efficiency is obtained after the catalyst reaches the Hg0 adsorption equilibrium. But in

5

Fig. 9, Hg0 adsorption on catalyst surface is unsaturated and there are some amounts of surface

6

active sites for SO2 giving SO3 which can improve Hg0 oxidation process through SO3 + O* + Hg0

7

→ HgSO4. Therefore, the higher Hg0 oxidation efficiency in Fig. 9 is owing to Hg0 adsorption. 100

200 ppm SO2 in

0

Hg Removal Efficiency ()

90 Adsorption Efficiency

80 70 60 50

Oxidation Efficiency

40

5% O2 in

30 20 10 0 0

20

40

8 9 10

60

80

100

120

Time (min)

Fig. 9 SO2 effects on Hg0 adsorption and oxidation

3.2.3 Hg0 oxidation efficiency in a relatively long term test 100

In situ thermal recovery

90 80

0

Hg oxidation efficiency

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

70

Co3O4-nanorods

60 50 40 30 20 10 0

11 12

0

500

1000

1500

2000

2500

3000

Time (min)

Fig. 10 Hg0 oxidation efficiency of Co3O4-nanorods for 2880 min and in situ thermal recovery 16


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 29

1

Hg0 oxidation efficiency of Co3O4-nanorods during 2880 min test and in situ thermal recovery

2

was investigated and the results are shown in Fig. 10. The catalytic performance is stable in 800

3

min and then it decreases slowly during the following test. Hg0 oxidation efficiency reduces to

4

about 72% after 2880 min. After in situ thermal recovery, the catalytic activity of Co3O4-nanorods

5

recovers to the original level which suggests that Co3O4-nanorods have a good renewable

6

property.

7

3.3 Reaction mechanism

8

To understand the reaction mechanism, XPS was performed to analyze the surface atomic

9

concentration and ratios of the Co3O4-nanorods after 2880 min reaction and regeneration. As

10

shown in Fig. 11 a, the peak around 102.5 eV is corresponding to SiO2 which may attribute to

11

impurities from Na2CO3 with SiO2 concentration of about 0.6%. The two peaks around 100.3 and

12

104.6 eV can be detected in Co3O4-nanorods after 2880 min reaction which can be ascribed to

13

HgO

14

oxidated to be HgO during the oxidation process

15

leading to less available surface active sites and a lower Hg0 oxidation efficiency. After in situ

16

thermal regeneration at 550 °C, most of HgO decomposed and the Hg0 oxidation efficiency

17

recovered to the original level.

56, 57.

The intensity of those two peaks sharply gets weak after regeneration. Hg0 was 58-60.

It can accumulate on the catalyst surface

17


Page 19 of 29

Hg 4f

Si 2p Co3O4-nanorods for 48h reaction 2+

Hg Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2+

Hg

Co3O4-nanorods for 48h reaction regeneration

Si 2p

2+

Hg

110

1

108

2+

Hg

106

104

102

100

98

Binding Energy (eV)

2

Fig. 11 Hg 4f XPS spectra of the Co3O4-nanorods for 2880 min reaction and regeneration

3

Fig. 12 shows the Co 2p and O1s XPS spectra of the Co3O4-nanorods for 2880 min reaction and

4

regeneration and Table 4 shows the surface atomic ratios. The Co3+/Co2+ and (Oα+Oγ)/Oβ ratios of

5

Co3O4-nanorods for 2880 min reaction were much lower than those of fresh Co3O4-nanorods. It

6

suggests that Co3+, Oα and Oγ take part in Hg0 oxidation process. Based on previous works,

7

chemisorbed oxygen and oxygen defect species were considered to be active sites for Hg0

8

oxidation which can react with Hg0 forming HgO

9

Co2+. And then Co2+ can be reoxidized to Co3+ by gaseous O2 to finish the reaction cycle

61.

During this process, Co3+ was reduced to 46, 62.

10

Therefore, Co3+/Co2+ redox system can greatly improve the Hg0 oxidation efficiency in the

11

presence of O2. Based on the analysis above, a probable mechanism of Hg0 oxidation could be

12

deduced. Hg0 was firstly adsorbed on the surface active sites forming Hg0 (ad) bonded with

13

surface active oxygen and then Hg0 (ad) was oxidized to be HgO. Finally, the consumed surface

14

active oxygen was replenished by gaseous O2. The reaction mechanism could be summarized by

15

the following reaction equations:

16

Hg0 + Oα → Hg(ad)-O

17

Hg0 + Oγ → Hg(ad)-O 18


Energy & Fuels

1

Hg(ad)-O → HgO

2

1/2O2 (gaseous) + * (adsorption sites) → Oα

3

Co3O3-□ (oxygen vacancy) + 1/2O2 (gaseous) → Co3O3-Oγ

4

Co3O3-Oγ→ Co3O3-Oβ → Co3O4

5

After regeneration, the peaks ascribed Oα get a greater intensity and the Oα/Oβ ratios of

6

Co3+/Co2+ increases. It suggests that the decomposition of HgO gives more available oxygen

7

vacancies which can capture gaseous oxygen forming more surface chemisorbed oxygen species.

Co

Co3O4-nanorods for 48h reaction recovery

Co

Co

800

795

Co

2+

790

785

O

O

Co3O4 nanorods for 48h reaction regeneration

O

3+

Co

780

O 1s

Co3O4-nanorods for 48h reaction

3+

Co

3+

2+

9

Co

3+

2+

Co

8

2+

O

B

Co 2p

Co3O4-nanorods for 48h reaction

Intensity (a.u.)

A

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 29

O

O

536

775

534

532

530

528

526

Binding Energy (eV)

Binding Energy (eV)

Fig. 12 XPS spectra of the Co3O4-nanorods for 2880 min reaction and regeneration, A Co 2p, B O 1s

10

Table 4 Surface atomic ratios of the catalysts Co3+/Co2+

Catalysts

(Oα+Oγ)/Oβ

Co3O4-nanorods for 2880 min reaction

0.42

2.33

Co3O4-nanorods for 2880 min reaction regeneration

0.54

3.01

11 12

4. Conclusions

13

Co3O4-nanorods exposing (220) facet were successfully synthetized. Compared to

14

Co3O4-nanoparticles, Co3O4-nanorods have a larger BET surface area leading to more surface

15

active sites. Raman and XPS show that Co3O4-nanorods have a much higher Co3+/Co2+ ratio

16

resulting in higher chemisorbed oxygen and oxygen defect species which play an important role in 19


Page 21 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

Hg0 oxidation process. Therefore, Co3O4-nanorods have an excellent Hg0 oxidation efficiency

2

which is more than 90% in the temperature range of 100-300 ºC. After 2880 min reaction, the Hg0

3

oxidation efficiency of Co3O4-nanorods reduces to about 72% but it recovers to the original level

4

after in situ thermal treatment at 550 ºC suggesting a good renewable property. A probable

5

reaction pathway was deduced based on XPS analysis of Co3O4-nanorods before and after

6

reaction. Hg0 firstly react with chemisorbed oxygen forming HgO and the consumed chemisorbed

7

oxygen is replenished by gaseous O2. Co3+/Co2+ redox system can improve the electron shift to

8

enhance the Hg0 oxidation efficiency in the presence of O2. The effects of flue gas components O2,

9

NO, H2O and SO2 on Hg0 oxidation efficiency are also investigated. O2 and NO have positive

10

effects while H2O and SO2 have negative effects on Hg0 removal process. But the Co3O4 nanorods

11

still have an efficiency of 75% even in the presence of 8% H2O and 200 ppm SO2.

12

Acknowledgements

13

We gratefully acknowledge the financial supports from the National Natural Science Foundation

14

of China (51408098), Liaoning Provincial Natural Science (20180510054), Foundation of

15

China the Program for Changjiang Scholars (T2012049), Education Department of the Liaoning

16

Province of China (LT2015007), the Fundamental Research Funds for the Central Universities

17

(DUT18JC45).

18

References

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Co3O4 nanorods with great amount oxygen vacancies for highly

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