Modified with Transition Metals (Cu, Fe, Ni, Co) as Efficient

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Efficiency and Sustainability

Modified with Transition Metals (Cu, Fe, Ni, Co) as Efficient Deoxidizers for O Removal from Low-Concentration Coal Bed Methane 2

Pingfeng Hu, Hongyan Pan, Qian Lin, Jianxin Cao, Yuhua Zhang, and Min Zhao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00261 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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Energy & Fuels

1

Modified with Transition Metals (Cu, Fe, Ni, Co) as

2

Efficient

3

Low-Concentration Coal Bed Methane

4

Pingfeng Hu1, Hongyan Pan1*, Qian Lin2*, Jianxin Cao2, Yuhua Zhang1, Min Zhao1

5

1

6

[email protected]

7

2

8

Technology, Guiyang 550025, China

9

ABSTRACT: Deoxidizers Na2S/AC was modified with transition metal ions (Co2+, Ni2+, Fe3+,

10

and Cu2+) by co-impregnation and step-by-step impregnation methods. Those deoxidizers were

11

characterized and tested for the removing of O2 from low-concentration coal bed methane

12

(CBM). Results showed that the effective deoxidizing amount of deoxidizers (QO2) and

13

conversion rate of Na2S ( C Na S ) of the deoxidizers are in the following orders: Co-Na2S/AC >

14

Ni-Na2S/AC > Fe-Na2S/AC > Na2S/AC > Cu-Na2S/AC. As compared to that of the deoxidizer

15

Na2S/AC, the content of reactive oxygen species (O£-) of the modified deoxidizers was enhanced

16

with the doping of transition metal ions (Co2+, Ni2+, Fe3+, and Cu2+). The more content of O£- on

17

deoxidizer was, the higher deoxidizing activity there was when the dispersion of Na2S and metal

Deoxidizers

for

O2

Removal

from

Department of Chemical Engineering, Guizhou University, Guiyang 550025, China. Email:

Key Laboratory of Guizhou Province for Green Chemical Industry and Clean Energy

2

ACS Paragon Plus Environment

1

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

ions of deoxidizers was higher. The deoxidizer Co-Na2S/AC had the largest content of O£- and

2

higher dispersion of Na2S and Co2+, and this made it the largest QO2 (54.88 mL/g) and CNa2S

3

(75.50%) at 200 oC. However, although the content of O£- of the deoxidizer Cu-Na2S/AC was

4

higher than that of the deoxidizer Na2S/AC, the deoxidizer Cu-Na2S/AC had the smallest QO2

5

and CNa2S due to the more severe agglomerated of S6 and the larger crystal of Cu(OH)2 on this

6

deoxidizer. The deoxidizer Co-Na2S/AC prepared by step by step impregnation method had

7

higher content of O£-, higher dispersion of active component Na2S and assistant Co2+, this made

8

it the higher deoxidizing activity compared with the deoxidizer Co-Na2S/AC(c) prepared by

9

co-impregnation method.

10

KEYWORDS: low concentration CBM; deoxidizer; Na2S; reactive oxygen species O£-; metal

11

ions.

12

1. Introduction

13

Most low-concentration coal bed methane (CBM) is usually discharged directly in many

14

mines due to the low concentration of CH4 (< 30%, volume fraction, similarly hereinafter),

15

which leads to the pollution of environment and the wasting of resource.1 In order to effectively

16

utilize those low-concentration CBM, it is necessary to separate and accumulate CH4.2,3

17

However, CH4 can be easily ignited in this process due to the higher concentration of O2 in

18

low-concentration CBM .4 Therefore, the remove of O2 from low-concentration CBM is the key

19

step before the effective use of them.5

20 21

Several methods, such as pressure swing adsorption,6,7 catalytic combustion,8,9 and chemical reduction10, have been used for the removing of O2 from CBM. Among them, chemical


2

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Energy & Fuels

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reduction is one of the most promising technologies because of its higher removal efficiency of

2

O2, energy saving process, and eco-friendly. Metal sulfides, such as Na2S and CaS etc, can be

3

used as deoxidizer for the remove of O2, and Na2S shows higher activity. Zhang et al.11 found

4

out that when temperature was lower than 500 oC, the deoxidizing activity of Na2S was higher

5

than that of CaS. However, the outer part of Na2S particles was gradually covered by Na2SO4

6

(Na2S + O2→Na2SO4) with the reaction time increasing, which inhibited the continue reaction of

7

O2 and Na2S, and decreased the conversion rate of Na2S. In order to increase the contact area

8

between Na2S and O2 to enhance the conversion rate of Na2S, Na2S was dispersed onto activated

9

carbon (AC) with higher surface area by impregnation methods in our previous research. With

10

this method, Na2S conversion has been slightly improved, but it is still low. Indicating that just

11

increasing the contact area of Na2S and O2 can’t effectively solve that problem.

12

The ground state of O2 is triplet states and the bond length and bond energy of O=O of O2

13

are 1.21 Å and 494 kJ/mol, respectively. Relatively large activation energy is required for the

14

breaking of O=O bond of O2 molecule. In comparison with O2, reactive oxygen species O£- (O-,

15

O2- and O22-) have longer bond length (1.28Å for O2- and 1.49Å for O22-).12 It is suggested that

16

the longer the bond length, the smaller bond energy.13 Thus, the bond energy of O=O in O£- is

17

lower than that in O2, and smaller activation energy is required to break O=O bond of O£- in

18

reaction of Na2S with O£-.

19

Conversion of O2 to O£- to obtain more content of O£- in deoxidizers is the key to improve

20

the activity of deoxidizers. In order to obtain more content of O£-, metal oxide or metal ions was

21

added into other metal oxide in the previous reports. Xie14 indicated that abundant reactive

22

oxygen species can be generated when CuO was dissolved into CeO2 lattice. Hashemi et al.15


3

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

used MnCl2 to immobilize on montmorillonite and founded that the oxidation rate can be

2

accelerated because MnCl2 can activate oxygen molecules. Wang et al.16 indicated that Fe2+

3

could activate oxygen molecules by electron transfer to increase reaction rate.

4

Thus, it can be speculated that the amount of O£- could be enhanced if the deoxidizer was

5

doped with transition metal. In this work, transition metal ions (Co2+, Ni2+, Fe3+, and Cu2+) with

6

different ability to loss electrons were selected as assistant to prepare deoxidizers by step-by-step

7

impregnation method, in which the Na2S and AC were used as active component and carrier

8

respectively. The deoxidizers were characterized by TG, XRD, XPS, and SEM-EDS and tested

9

for the removing of O2 from low-concentration CBM in fixed reactor at 200 oC. Moreover, the

10

effect of preparation methods on the deoxidizing activity of the deoxidizer M-Na2S/AC (M = Co

11

and Ni) was analysis and discussed in this work.

12

2. Experiment

13

2.1 Deoxidizers Preparation

14

Pretreatment of AC: 40-60 mesh coconut shell activated carbon (AC) with the BET

15

surface area of 860.3 m2/g provided from Tangshan Chenhui Carbon company, China, were

16

washed by hydrochloric acid, sodium hydroxide, and deionized water for several times until the

17

water solution was neutral, and then dried at 80 oC for 24 h.

18

Preparation of deoxidizer Na2S/AC: First, 30 mL 2.78 mol/L Na2S solution was added

19

into a conical flask of 50 g AC. The slurries were impregnated in a shaking table at 20 oC for 1 h,

20

and dried at 100 oC for 5 h. The obtained deoxidizer was denoted as Na2S/AC, in which the

21

loading amount of Na2S was 13%.


4

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1

Energy & Fuels

Preparation of the deoxidizers M-Na2S/AC: The deoxidizers were prepared by

2

step-by-step impregnation method. First, 30 mL 0.18 mol/L solution of Fe(NO3)3 and M(NO3)2

3

(M= Co, Ni, and Cu) was added into a conical flask of 50 g AC respectively. The slurries were

4

impregnated for 24 h at 30 oC and dried at 100 oC for 12 h. Then, it was added into 2.78 mol/L 30

5

mL Na2S solution, after impregnated and dried, the deoxidizers were obtained and denoted as

6

M-Na2S/AC, in which the loading amount of metal ions and Na2S is 2 and 13%, respectively.

7

The heat treatment conditions of the deoxidizers M-Na2S/AC is the same as the deoxidizer

8

Na2S/AC.

9

Preparation of the deoxidizers Co-Na2S/AC(c) and Ni-Na2S/AC(c): The deoxidizers

10

were prepared by co-impregnation method. First, the mixture solution of 30 mL 0.18 mol/L

11

M(NO3)2 (M = Co and Ni) and 30 mL 2.78 mol/L Na2S was added into a conical flask of 50 g

12

AC. Then the slurry was impregnated and dried at the same condition presented above. The

13

obtained deoxidizers were denoted as Co-Na2S/AC(c) and Ni-Na2S/AC(c) with loading amount

14

of Na2S of 13% and metal ions of 2%.

15

2.2 Characterization of Deoxidizers

16

TG/DTG analysis: The thermal behavior of deoxidizers was evaluated by thermalanalyzer

17

(Netzsch Co, GER) in the temperature range of 40-840 °C with the heating rate of 10 °C/min,

18

using air as carrier gas.

19

XPS analysis: The valance of metal ions and the content of O£- of deoxidizers were

20

evaluated by X-ray photoelectron spectra Escalab 250 (XPS, Thermo Co., USA) with nonmono

21

chromatic Al Ka (1486.6 eV) at power of 150 W. Standard C1s binding energy is 284.8 eV.


5

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

SEM-EDS: The morphologies of deoxidizers were measured by scanning electron

1 2

microscopy (SEM, Hitachi S-4800 Co, JPN). The dispersion details of deoxidizers were

3

measured by Energy dispersive X-ray spectrometer (EDS, Quantax400, Bruker Co., Germany)

4

coupled with the microscope chamber.17 XRD: The crystal structure of deoxidizers were characterized by XRD instrument (D8,

5 6

Brookfield, G.E.R) using a Kα radiation ( λ = 0.15432 nm). The scanning speed was 5o/min and

7

the tube current was 10 mA. Elemental analysis: The content of C, S, and Co in deoxidizers were quantitatively analyzed

8 9

by Element analyzer (Vario EL Ⅲ, Elementar, German), using 99.999% He as protective gas

10

and 99.999% O2 as oxidizing gas.

11

2.3 Activity Test Firstly, 70 g deoxidizer M-Na2S/AC was put in the middle of fixed bed reactor with column

12 13

length and inner diameter of 60 and 2 cm respectively, and then packed with quartz wool.

14

Secondly, the fixed bed reactor was increased to 100 oC from room temperature at a rate of 10

15

o

16

was continue increased to 200 oC and at this time the flow gas of N2 was changed to 50 mL/min

17

CBM (CH4 20%, O2 16% and N2 64%, vol. %). The composition of exit gas stream from the

18

fixed bed was determined by online gaschromatography (GC9560, Shanghai Hua’ai

19

ChromatographCom, China) equipped with thermal conductivity detector (TCD) and mass

20

spectrometer (QGA, Hiden, Britain).

21

C/min in N2 flow of 100 mL/min, and then dried at this temperature for 2 h. Then the fixed bed

Effective deoxidizing amount of deoxidizers ( Q O2 ) and the conversion rate of Na2S ( CNa2S )


6

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Page 7 of 24

1

can be calculated by the follow formulas:

（ c0 − ct） × Q × t

QO =

2

m

2

(1)

（ c0 − ct） × Q × t

C Na S =

3

V

2

(2)

4

where c0 and ct are the O2 concentration of inlet CBM and effluent gas (vol.%), respectively,

5

t is the breakthrough time of O2 (Ct = 0.001), Q is the volumetric flow rate of CBM (mL/min), m

6

is the weight of deoxidizer (g), V is the volume of O2 consumed that S2- of deoxidizer completely

7

convert into SO42- (mL).

8

3. Results and Discussion

9

3.1 Effect of Metal Ions on the Deoxidizing Activity of the Deoxidizer Na2S/AC

10

3.1.1 TG/DTG Analysis

25

a DTG (mg/min)

Fe-Na2S/AC

15

Co-Na2S/AC 10

Cu-Na2S/AC

5

0 200

Na2S/AC Fe-Na2S/AC Co-Na2S/AC

-0.5

Ni-Na2S/AC -1.0

Cu-Na2S/AC

-1.5

Ni-Na2S/AC

0

b

0.0

Na2S/AC

20

Mass (mg)

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

400

600

-2.0

800

0

200

T (°C)

400

600

800

T (°C)

11

Figure 1. TG (a) and DTG (b) curves of deoxidizer M-Na2S/AC modified by different metal ions.

12

Analysis conditions: 10 oC/min heating rate, 20 mL/min air flow rate, 40-840 °C.


7

Energy & Fuels

Figure 1 shows the TG and DTG curves of the modified deoxidizers M-Na2S/AC (M= Cu,

1 2

Fe, Ni, and Co). As shown in Figure 1a, these deoxidizers exhibit two weight-loss steps in TG

3

curves in the temperature range of 40-840 °C. The first weight-loss step that occurs below

4

200 °C is the evaporation of free water, and the second weight-loss step that occurs between 300

5

and 600 °C is the oxidation reaction between deoxidizers and oxygen of air. The

6

DTG

curves

of

those

deoxidizers

(Figure

1b)

shows

that

7

the temperature of maximum weight loss rate of all deoxidizers in first weight-loss step is almost

8

the same, whereas, the temperature of maximum weight loss rate of the modified deoxidizers

9

M-Na2S/AC in second weight-loss step is lower than that of the deoxidizer Na2S/AC. That means

10

the doping of transitional metal ions into deoxidizer Na2S/AC has no effect on the evaporation

11

rate of free water but decreases the oxidation reaction temperature of deoxidizers.

12

3.1.2 XRD Analysis

∆ C ∆

⊕ S6

∆ Cu(OH)2

⊕ 

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 8 of 24



⊕







⊕

Cu-Na2S/AC Fe-Na2S/AC







Ni-Na2S/AC



Co-Na2S/AC Na2S/AC

10

13 14

20

30

40

50

60

70

80

90

100

o

2θ ( )

Figure 2. XRD patterns of deoxidizers M-Na2S/AC modified by different metal ions.

15

Figure 2 shows the XRD patterns of the deoxidizers M-Na2S/AC. It can be seen that all

16

deoxidizers exhibit two visible diffraction peaks bread at 2 θ of 26° and 43°, which represent the


8

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Energy & Fuels

1

characteristic diffraction peaks of amorphous carbon (JCPDS05-0625).18 For the two deoxidizers

2

Na2S/AC and Co-Na2S/AC, no new diffraction peaks can be detected besides the structure of

3

amorphous carbon, that means the particle size of active component Na2S and assistant Co is

4

very small and below the range of the detection of XRD. However, for the three deoxidizers

5

Cu-Na2S/AC, Fe-Na2S/AC and Ni-Na2S/AC, a new diffraction peak at 2 θ of 8.8° that represents

6

of S6 (JCPDS72-2402) can be detected besides the diffraction peaks of amorphous carbon,

7

especially for the deoxidizer Cu-Na2S/AC, the diffraction peak intensity of S6 is the strongest.

8

The stronger the diffraction peak intensity is, the larger the particle size of crystal,19 which

9

indicates the aggregation of Na2S on the deoxidizer Cu-Na2S/AC is more severe than that on

10

Fe-Na2S/AC and Ni-Na2S/AC.

11

Moreover, other two diffraction peaks can also be detected at 2 θ of 17° and 24° (form

12

Cu(OH)2 (JCPDS03-0310)) on the deoxidizer Cu-Na2S/AC. The formation of Cu(OH)2 is due to

13

the reaction between Cu2+ and OH-, in which OH- is generated from the hydrolysis of some S2- in

14

Na2S solution. The loss of Cu2+ of the deoxidizer Cu-Na2S/AC weakens its activating ability

15

towards oxygen.

16

3.1.3 XPS Analysis

a

c

b Na2S/AC O1s

AC O1s

Cu-Na2S/AC O1s C=O

O-H

C=O

C=O

O-H

O-H

O£-

NaKLL

524

526

528

530

532

534

B.E.(eV)

536

538

540

542

526

528

530

532

534

536

NaKLL

538

540

542

B.E.(eV)

526

528

530

532

534

B.E.(eV)


9

536

538

540

542

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

e Ni-Na2S/AC

d Fe-Na2S/AC

f Co-Na2S/AC

O1s

O1s

O1s

C=O

C=O

C=O O-H

O£-

526

528

Page 10 of 24

530

532

534

O-H

O-H

O£-

NaKLL

536

538

540

542

526

528

530

532

B.E.(eV)

534

O£-

NaKLL

536

NaKLL

538

540

542

526

528

530

532

534

536

538

540

542

B.E.(eV)

B.E.(eV)

Figure 3. XPS spectra of the deoxidizers AC, Na2S/AC and M-Na2S/AC; M=Cu, Fe, Ni, Co; O£-: the active oxygen species of O-, O2- or O22-. 1

Table 1. XPS parameters of O 1s Samples

Key type

C=O

O-H

O£-

BE/eV

532.58

533.0

-

RC/%

50.97

49.03

-

BE/eV

531.2

531.8

-

RC/%

65.44

34.56

-

BE/eV

531.4

532.1

533.94

RC/%

36.02

54.08

9.9

BE/eV

531.6

532.1

533.46

RC/%

50.29

32.81

16.90

BE/eV

531.5

532.1

533.3

RC/%

65.54

14.46

20.0

AC

Na2S/AC

Cu-Na2S/AC

Fe-Na2S/AC

Ni-Na2S/AC


10

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Energy & Fuels

BE/eV

531.6

532.4

533.3

RC/%

40.06

37.54

22.4

Co-Na2S/AC

1

Figure 3 shows XPS spectra of O 1s of the AC and deoxidizers M-Na2S/AC. It can be seen

2

that just one peak exists on AC. However, when Na2S is loaded on AC, a new peak of NaKLL

3

located around 536 eV can be detected on all deoxidizers. The generation of this new peak is due

4

to the disturbing effect of Na ion. In addition, compared with the O 1s spectra of the deoxidizer

5

Na2S/AC (Figure 3b), O 1s spectra of other deoxidizers modified with metal ions is asymmetric,

6

and a shoulder peak is present on higher binding energy of 533-534 eV, corresponding to the

7

unreduced active oxygen species O£- (O-, O2- and O22-).20

8

According to those pictures, O 1s can be fitted into two peaks for the samples of AC and

9

Na2S/AC, corresponding to the groups of C=O and O-H.21,22 However, O 1s can be fitted into

10

three peaks for those deoxidizers modified by metal ions, corresponding to the groups of C=O,

11

O-H and O£-. Each peak area and its corresponding binding energy are listed in Table 1. The data

12

in Table 1 shows that the content of O£- on those deoxidizers is in the following orders:

13

Co-Na2S/AC > Ni-Na2S/AC > Fe-Na2S/AC > Cu-Na2S/AC > Na2S/AC. It indicates that the

14

content of reactive oxygen species O£- of the modified deoxidizers is enhanced when metal ions

15

Co2+, Fe3+, Ni2+, and Cu2+ are loaded onto the deoxidizer Na2S/AC due to the fact that the

16

valence shell electrons of metal ions are easy to be lost. Those electrons were transferred to

17

oxygen molecule to form reactive oxygen species O£-. The deoxidizer Co-Na2S/AC has the

18

largest content of O£- is because Co2+ ion has the strongest ability to loss electrons and it is

19

highly dispersed on this deoxidizer.

20

3.1.4 The Deoxidizing Activity of the Deoxidizers M-Na2S/AC


11

Energy & Fuels

Na2S/AC

a

12

Ni-Na2S/AC

10

Co-Na2S/AC

20 15

8 6

Na2S/AC

10

4 2 0

0

100

200

t (min)

300

c

Fe-Na2S/AC

5

Ni-Na2S/AC Co-Na2S/AC

0

400

0

Na2S/AC

100

200

300

t (min)

d

Cu- Na2S/AC

100

Cu- Na2S/AC

400

N2

Fe-Na2S/AC Ni-Na2S/AC Co-Na2S/AC

90

80

Faraday torr (a.u)

Volume Fraction of O2 (%)

Fe-Na2S/AC

b

25

Cu- Na2S/AC

14

Volume Fraction of CH4 (%)

16

Volume Fraction of N2 (%)

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 12 of 24

CH4 N2

Na2S/AC N2

N2CH4

70

60 0

100

200

300

10

400

t (min)

Co Na2S/AC __

20

30

mass amu

40

50

1

Figure 4. Breakthrough curves of O2 (a), N2 (b) and CH4 (c) on deoxidizer M-Na2S/AC modified

2

by different metal ions. Reaction conditions: 200 oC reaction temperature and 1 atm 50 mL/min

3

CBM (CH4 20%, O2 16% and N2 64%, vol. %) flow rate.

4

Table 2. Deoxidizing amount of O2 from CBM and the conversion rate of Na2S on deoxidizers

5

modified by metal ions Deoxidizers

t (min)

Q O2 (mL/g)

CNa2S (%)

Na2S/AC

287.8

40.75

56.06

Cu-Na2S/AC

278.3

39.40

54.21

Fe-Na2S/AC

313.2

44.33

61.01

Ni-Na2S/AC

333.1

46.88

64.88

Co-Na2S/AC

387.6

54.88

75.50


12

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1

Energy & Fuels

Figure 4 shows the curves of the concentration of O2 (Figure 4a), N2 (Figure 4b) and CH4

2

(Figure 4c) in effluent gas as a function of reaction time of the deoxidizers M-Na2S/AC. In

3

compared with Na2S/AC as shown in Figure 4a, the deoxidizing curves (breakthrough curves of

4

O2) of deoxidizers Ni-Na2S/AC, Fe-Na2S/AC and Co-Na2S/AC shift to right, and the deoxidizing

5

time (breakthrough time of O2) increase, especially for the deoxidizer Co-Na2S/AC whose

6

deoxidizing time is the longest (387.6 min). However, for the deoxidizer Cu-Na2S/AC, its

7

deoxidizing curves shifts to left and deoxidizing time decreases.

8

As shown in Figure 4b, the concentration of CH4 gradually increases from 0 to 24% on

9

different deoxidizers when reaction time increases from 0 to 30min, and then remains unchanged

10

within the whole deoxidizing time. Meanwhile, the concentration of N2 (Figure 4c) gradually

11

decreases from 100 to 76% and then remains unchanged in the same time frame. This is because

12

AC who is the carrier of deoxidizer has the adsorption ability towards CH4 within 30 min.

13

Moreover, it can be seen that for all deoxidizers during the effective deoxidizing time, the

14

concentration of CH4 and N2 in the effluent gas can increase to 24 and 76%, respectively, it is 20

15

and 19% higher, respectively, than that of the inlet concentration of CH4 and N2 in inlet CBM.

16

This is because O2 is removed from low-concentration CBM.

17

The effective deoxidizing amount of O2 of deoxidizers QO2 and the conversion rate of Na2S

18

CNa2S are listed in Table 2, which are calculated by equation (1) and (2) based on the data of

19

Figure 4a. It can be seen that QO2 and CNa2S of the deoxidizers are in the following orders:

20

Co-Na2S/AC > Ni-Na2S/AC > Fe-Na2S/AC > Na2S/AC > Cu-Na2S/AC, indicating that the

21

loading of Co2+, Ni2+ and Fe3+ enhance the deoxidizing activity of deoxidizers, especially for the

22

deoxidizer Co-Na2S/AC with the largest QO2 (54.88 mL/g) and largest CNa2S (75.50%), which is


13

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

34.67 and 34.67% higher than that of Na2S/AC respectively. This is because the loading of Co2+,

2

Ni2+ and Fe3+ ions onto the deoxidizers can increase the content of reactive oxygen species O£-,

3

especially for the deoxidizer Co-Na2S/AC, it has the largest content of O£-. Moreover, active

4

component Na2S is highly dispersed on those deoxidizers.

5

Since the O=O bond in O2- and O22- of O£- is easier to be broken in compared with the O=O

6

bond of O2, thus the reaction between Na2S and O£- is easier to be taken place. The higher

7

content of O£- on deoxidizers is, the deoxidization reaction is easier and the activity of the

8

deoxidizer is higher. The deoxidizer Co-Na2S/AC has the largest content of O£- and higher

9

dispersion of Na2S, results in the highest deoxidizing activity in our research.

10

However, the loading of Cu2+ onto the deoxidizer reduce its deoxidizing activity although

11

the content of O£- on this deoxidizer Cu-Na2S/AC is larger than that on Na2S/AC. This is because

12

the more severe agglomerated of S6 and the formation of crystal of Cu(OH)2 are detected on this

13

deoxidizer, and the agglomerated of S6 can reduced the reaction between Na2S and O2(O£-).

14

It has been known that the carrier AC can react with O2 at a certain temperature. In order to

15

determine whether the metal ions or the carrier AC has a positive role on the deoxidizing activity

16

of the deoxidizers, the effluent composition is determined by mass spectrum for the two

17

deoxidizers Na2S/AC and Co-Na2S/AC and the data are shown in Figure 4d. It can be seen that

18

N2 and CH4 but CO, CO2 and O2 can be seen for the two deoxidizers, which indicates carrier AC

19

can’t react with O2 of low-concentration CBM at the temperature of 200 oC. Thus, the higher

20

deoxidizing activity of the deoxidizers is due to the doping of metal ions onto the Na2S/AC other

21

than the reaction of carrier AC and O2.


14

Page 14 of 24

Page 15 of 24

1

3.2 Effect of Preparation Methods on the Activity of the Deoxidizers Co-Na2S/AC and

2

Co-Na2S/AC(c)

3

3.2.1 TG/DTG Analysis 0.2

a

12

0.0 10

b

-0.2

Co-Na2S/AC

8

DTG (mg/min)

Mass (mg)

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

-0.4

Co-Na2S/AC

-0.6

6

Co-Na2S/AC(c)

4

-0.8 -1.0

2

Co-Na2S/AC(c)

-1.2

0

-1.4 0

200

400

600

800

0

200

400

600

800

1000

T (°C)

T (°C)

4

Figure 5. TG (a) and DTG (b) curves of the deoxidizers Co-Na2S/AC and Co-Na2S/AC(c).

5

Analysis conditions: 10 oC/min heating rate, 20 mL/min air flow rate, 40-840 °C.

6

Figure 5 shows the TG and DTG curves of the deoxidizers Co-Na2S/AC(c) and

7

Co-Na2S/AC prepared by co-impregnation method and step by step impregnation method,

8

respectively. It indicates that the temperature of maximum weight loss rate of the two deoxidizers

9

in the first weight-loss step is almost the same. However, for the second weight-loss step,

10

the temperature of maximum weight loss rate of the deoxidizer Co-Na2S/AC is 435 °C, which is

11

46 °C lower than that of the deoxidizer Co-Na2S/AC(c). That means the deoxidizer Co-Na2S/AC

12

can decrease the oxidation temperature and enhance its catalytic oxidation rate.

13

3.2.2 XRD Analysis


15

Energy & Fuels



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



C ⊕ S6 ∆ CoSO4

•

Co(OH)2 ♦Na2S⋅9H2O

Page 16 of 24

• ⊕

♦ ♦ 

Co-Na2S/AC(c) 



Co-Na2S/AC 10

20

30

1 2

40

2θ (0)

50

60

70

80

Figure 6. XRD patterns of the deoxidizers Co-Na2S/AC and Co-Na2S/AC(c). Figure 6 shows the XRD patterns of the deoxidizers Co-Na2S/AC and Co-Na2S/AC(c). By

3 4

comparison with Co-Na2S/AC, some new diffraction peaks at 2 θ of 8.8°, 20°, 33° and 34° can

5

be detected on the deoxidizer Co-Na2S/AC(c). Among them, a diffraction peak at 2 θ of 8.8° and

6

20° is considered as S6 and Co(OH)2 (JCPDS01-0357) respectively, and the diffraction peak at 2

7

θ of 33° and 34° is denoted as Na2S·9H2O (JCPDS03-0745). That means active component

8

Na2S and assistant Co2+ become reunited on the deoxidizer Co-Na2S/AC(c).

9

3.2.3 SEM-EDS Analysis

S

Co

C

S

Co

C

A

Co-Na2S/AC

B

Co-Na2S/AC) (c)


16

Page 17 of 24 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

Energy & Fuels

Figure 7. SEM-EDS imagines of deoxidizer Co-Na2S/AC and Co-Na2S/AC(c) Figure 7 A and B shows the SEM-EDS mapping of the deoxidizer Co-Na2S/AC and

2 3

Co-Na2S/AC (c). It can be seen that the amount of Co per unit area of the deoxidizer

4

Co-Na2S/AC(c) is smaller than that of the deoxidizer Co-Na2S/AC. Table 3 shows the elements

5

analysis of C, S and O of the two deoxidizers, and the data shows that the contents of Co on the

6

deoxidizer Co-Na2S/AC(c) is smaller than that on Co-Na2S/AC. This is due to the aggregation of

7

Co and it is verified by XRD analysis. Table 3. Elemental analysis of Co-Na2S/AC and C o-Na2S/AC(c)

8

Element content (W %) Samples

9

C

S

Co

Co-Na2S/AC(c)

65.99

3.47

0.36

Co-Na2S/AC

64.48

3.67

0.42

3.2.4 XPS Analysis b

a

Co2p3/2

Co

O1s

Co-Na2S/AC(c)

C=O

Co-Na2S/AC(c)

Co2p1/2 Co 2p1/2 II

CoIICo2p3/2

CoII2p1/2

CoIICo2p3/2

O-H O£-

526

528

530

532

534

NaKLL

536

538

540

542

Co-Na2S/AC

810

800

790

Co-Na2S/AC

O-H

C=O

526

528

530

532

534

CoII2p1/2

CoIII2p1/2

NaKLL

536

538

540

542

810

B.E.(eV)

800

B.E.(ev)

790

780

Figure 8. XPS spectra of the deoxidizers Co-Na2S/AC and Co-Na2S/AC(c) 10

Table 4. XPS parameters of O 1s and Co 2p Key Samples

770

CoIIICo2p3/2

CoIICo2p3/2

Co2p1/2 O£-

780

Co2p3/2

C=O

O-H

O£-

type


17

Co2+

Co3+

770

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 24

BE/eV

531.6

532.4

533.3

796.46,781.7

795.31,780.3

RC/%

40.06

37.54

22.40

45.87

54.13

BE/eV

531.5

532.1

533.08

798,782,783.6

-

RC/%

50.62

27.78

21.60

100

0

Co-Na2S/AC

Co-Na2S/AC(c)

1

Figure 8 shows XPS spectra of Co 2p and O 1s of the two deoxidizers Co-Na2S/AC(c) and

2

Co-Na2S/AC, corresponding peak area and binding energy are listed in Table 4. It can be seen

3

that Co 2p3/2 binding energy of the deoxidizer Co-Na2S/AC(c) (782.0 eV) is 0.3 eV higher than

4

that of the deoxidizer Co-Na2S/AC, which indicates there is a stronger interaction between Co2+

5

and other components, and a more stable material can be formed on this deoxidizer. The fitting

6

data of Co 2p3/2 of the deoxidizer Co-Na2S/AC(c) shows that only Co2+ is present on this

7

deoxidizer. Meanwhile, XRD analysis indicates that Co(OH)2 is present on the deoxidizer

8

Co-Na2S/AC(c). Thus, one can infer that the stable material on this deoxidizer Co-Na2S/AC(c) is

9

Co(OH)2. For the deoxidizer Co-Na2S/AC, the fitting data of Co 2p3/2 shows that Co2+ and Co3+

10

11

co-exist on this deoxidizer. The fitting data of O 1s of the two deoxidizers shows that the content of O£- on the

12

deoxidizer Co-Na2S/AC is higher than that on the deoxidizer Co-Na2S/AC(c). This is because Co

13

is highly dispersed on the deoxidizer Co-Na2S/AC. However, for the deoxidizer Co-Na2S/AC(c),

14

the relatively stable of Co(OH)2 can be detected on this deoxidizer. Because Co2+ in Co(OH)2 of

15

the deoxidizer Co-Na2S/AC(c) is stable that could not provide electron to active oxygen

16

molecules to produce active oxygen species O£-, which results in the deoxidizer Co-Na2S/AC(c)

17

with less active oxygen species O£-.


18

Page 19 of 24

3.2.5 Influence of Preparation Method on the deoxidizing Activity of Oxidizer Na2S concentration of O2 in outlet stream(%)

1

16

Ni-Na2S/AC(c) Ni-Na2S/AC Co-Na2S/AC(c) Co-Na2S/AC

a

14 12 10 8 6 4 2 0 0

100

200

300

400

100

b


95

90

85

80

75

70 0

100

200

300

400

concentration of CH4 in outlet stream(%)

time (min) concentration of N2 in outlet stream(%)

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

25

c

20

15

10


5

0 0

time (min)

100

200

300

400

time (min)

2

Figure 9. Influence of preparation method on the M-Na2S/AC deoxidizing effect. Reaction

3

conditions: 200 oC reaction temperature and 1 atm 50 mL/min CBM (CH4 20%, O2 16% and N2

4

64%, vol. %) flow rate.

5

Table 5. Influence of preparation method on the M-Na2S/AC deoxidizing effect Deoxidizers

t (min)

QO2 (mL/g)

CNa2S (%)

Ni-Na2S/AC(c)

309.09

43.76

60.20

Ni-Na2S/AC

330.59

46.81

64.39

Co-Na2S/AC(c)

356.24

50.44

69.39

Co-Na2S/AC

392.07

55.51

76.37


19

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

Figure 9 shows the curves of the concentration of O2 (Figure 9a), N2 (Figure 9b) and CH4

2

(Figure 9c) in effluent CBM as a function of reaction time under the help of deoxidizers

3

Co-Na2S/AC and Co-Na2S/AC(c), the deoxidizing activity of the deoxidizer Ni-Na2S/AC and

4

Ni-Na2S/AC(c) as a comparison is also shown in this figure. Table 5 shows the QO2 and CNa2S of

5

these deoxidizers. It can be seen that the effective deoxygenating time t, QO2 and CNa2S of the

6

deoxidizer Co-Na2S/AC is higher than that of the deoxidizer Co-Na2S/AC(c). This is because the

7

former deoxidizer has a more content of reactive oxygen species O£- and higher dispersion of

8

active component Na2S and assistant cobalt ions. The deoxidizer Co-Na2S/AC(c) with weaker

9

deoxidizing activity is due to the aggregation of S6 and Co(OH)2, and the less content of O£-.

10

11

4. Conclusions (1) The doping of Co2+, Ni2+, and Fe3+ onto the deoxidizer Na2S/AC can improve the

12

deoxidizing activity, especially for the deoxidizer Co-Na2S/AC, which has the largest QO2 (54.88

13

mL/g) and CNa2S (75.50%). The transition metal ions modified deoxidizer Na2S/AC has higher

14

deoxidizing activity than Na2S/AC, due to the larger content of O£- on the deoxidizers. The

15

deoxidizer Co-Na2S/AC has higher content of O£- and higher dispersion of active component

16

Na2S, this makes it the highest deoxidizing activity. The loading of Cu2+ onto the deoxidizer

17

Na2S/AC decreases the deoxidizing activity although the content of O£- on this deoxidizer

18

Cu-Na2S/AC is larger than that on Na2S/AC. This is because the more severe agglomerated of S6

19

and the formation of crystal of Cu(OH)2 are present on this deoxidizer Cu-Na2S/AC.

20

(2) In comparison with the deoxidizing Co-Na2S/AC(c), the deoxidizer Co-Na2S/AC

21

prepared by step by step impregnation method has higher deoxidizing activity. This is because


20

Page 20 of 24

Page 21 of 24 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

the content of O£- on the deoxidizer Co-Na2S/AC is larger than that on Co-Na2S/AC(c), and

2

meanwhile the dispersion of active component Na2S and assistant Co2+ is also higher on this

3

deoxidizer.

4

AUTHOR INFORMATION

5

Corresponding Author

6

*Tel.:

+86-851-83604936.

7

[email protected].

8

Funding

Fax:+86-851-3625867.

E-mail:

[email protected].

9

This work was supported by the National Natural Science Foundation of China (No.

10

21366008), Foundation of Guizhou Provincial ministry of education (No. (2014)267), Science &

11

Technology Foundation of Guizhou Province (No. (2014)2008), National Natural Science

12

Foundation of China (Grant No. 21666007) and Scientific and Technological Innovation Talents

13

Team of Guizhou (No. 2018-5607).

14

Notes

15

16

The authors declare no competing financial interest. References

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(2) Pan, H. Y.; Zhao, J. Y.; Lin, Q; Cao, J. X.; Zheng, B. L. Preparation and characterization of activated carbons from bamboo sawdust and its application for CH4 selectivity adsorption from a CH4/N2 system. Energy Fuels. 2016, 30, (12), 10730-10738. (3) Su, Y. M.; Xu, S. P.; Wang, J. F.; Liu, W. Z.; Xiao, R. L.; Ouyang, S. B. Influence of micropore structure of activated carbons on their selective adsorption of CH4 from CH4/N2 mixture. Natural Gas Industry. 2013, 33, (3), 89-94. (4) Wang, G. Study on detection of gases explosiveness in coal mine fire. Liaoning Project Technology University. 2008. (5) Zhao, N. Research progress of purification technology in low-concentration CBM and its application. Guangdong Chemical Industry. 2016, 43, (20), 133-135. (6) Kulish, S.; Swank, R. P. Rapid cycle pressure swing adsorption oxygen concentration method and apparatus: US 5827358 A. 1998. (7) Tedford, R. A.; Jr. Concentrating the oxygen in air using absorbing beds; medical equipment; efficiency: US. (8) Zhang, Q.; Wu, X. P.; Zhao, G.; Li, Y.; Wang, C.; Liu, Y.; Gong, X. Q.; Lu, Y. High-performance Pd Ni alloy structured in-situ on monolithic metal-foam for coal bed methane deoxygenation via catalytic combustion. Chem. Commun. 2015, 51, (63), 126-136. (9) Zhang, Q. F.; Li, Y. K.; Chai, R. J.; Zhao, G. F.; Ye, L.; Yong, L. Low-temperature active, oscillation-free Pd Ni(alloy)/Ni-foam catalyst with enhanced heat transfer for coal bed methane deoxygenation via catalytic combustion. Appl. Catal., B (Environmental). 2016, 187, 238-248. (10) Tian, F.; Zhang, T.; Zhang, Y.; Zhang, H. R.; Li, X. L.; Sun, Y. L.; Zhang, Y. F. Study on Na2S deoxidation of coal bed methane in low gassy mine. Coal Sci. Techno. 2011. 39, (07), 124-128.


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(11) Zhang, Y.; Zhang, Y. F.; Zhang, G. J. Deoxygenation characteristic of sulfide oxidation in the process of oxygen-bearing coal mine methane. Coal Convers. 2009, 32, (1), 68-71. (12) Placa, S. J. L.; Ibers, J. A. Structure of IrO2Cl(CO)(P(C6H5)3)2, the oxygen adduct of a synthetic reversible molecular oxygen carrier1. J. Am. Chem. Soc. 1965, 87, (12), 2581-2586. (13) Pauling, L. The dependence of bond energy on bond length. J. Phys. Chem. 1954, 58, (8), 662-666. (14) Xie, H. M.; Du, Q. X.; Li, H.; Zhou, G. L.; Chen, S. M.; Jiao, Z. J.; Ren, J. M. Catalytic combustion of volatile aromatic compounds over CuO-CeO2, catalyst. Korean J. Chem. Eng. 2017, 1-8. (15) Hashemi, M. M.; Akhbari, M.; Karimi-Jaberi Z. Solid phase regeneration of carbonyl compounds by oxidative cleavage of carbon-nitrogen double bonds with molecular oxygen at room temperature. Lett. Org. Chem. 2006, 3, (2), 121-122 (16) Wang, L.; Cao, M. H.; Ai, Z. H.; Zhang, L. Z. Design of a highly efficient and wide PH electro-Fenton oxidation system with molecular oxygen activated by ferrous-tetrapolyphosphate complex. Environ. Sci. Technol. 2015, 49, (5), 3032-3039. (17) Chen, Z.; Pan, H. Y.; Lin, Q.; Zhang, X.; Xiao, S.; He, S. The modification of Pd core–silica shell catalysts by functional molecules (KBr, CTAB, SC) and their application to the direct synthesis of hydrogen peroxide from hydrogen and oxygen. Catal. Sci. Tech., 2017, 7, 1415-1422. (18) Fu, T.; Lv, J.; Li. Z. Effect of carbon porosity and cobalt particle size on the catalytic performance of carbon supported cobalt fischer–tropsch catalysts. Ind. Eng. Chem. Res. 2014, 53, (4), 1342-1350.


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(19) Pan, H. Y.; He, Z. Y.; Lin Q.; Liu, F.; Li, Z. The effect of copper valence on catalytic combustion of styrene over the copper based catalysts in the absence and presence of water vapor. Chinese J. Chem. Eng. 2016, 24, (4), 468-474. (20) Smith, M.; Scudiero, L.; Espinal, J.; Mcewen, J. S. Garcia-Perez, M. Improving the deconvolution and interpretation of XPS spectra from chars by ab initio calculations. Carbon, 2016, 110, 155-171. (21) Zhu, M. P.; Zhou, K. B.; Sun, X. D.; Zhao, Z. X.; Tong, Z. F.; Zhao, Z. X.; Hydrophobic N-doped porous biocarbon from dopamine for high selective adsorption of p-Xylene under humid conditions. Chem. Eng. J. 2017, 317, 660-672. (22) Hu, P.; Liang, X. P.; Yaseen, M.; Sun, X. D.; Tong, Z. F.; Zhao, Z. X.; Zhao, Z. X. Preparation of

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mechano-chemical method for (CHO-Cl-)-VOCs competitive adsorption in humid environment. Chem. Eng. J. 2018, 332, 608-618.


24

Modified with Transition Metals (Cu, Fe, Ni, Co) as Efficient

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