Performance of Bimetallic Additives (Fe-Co, Mn-Co, Cu-Co and Zn-Co

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Performance of Bimetallic Additives (Fe-Co, Mn-Co, CuCo and Zn-Co) Modified NaS/AC Deoxidizers towards Removal of O from Low Concentration Coalbed gas 2

2

Yuhua Zhang, Hongyan Pan, Qian Lin, Fuxin Liu, Guoxiang Zhang, and Pingfeng Hu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00675 • Publication Date (Web): 31 May 2019 Downloaded from http://pubs.acs.org on June 7, 2019

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Energy & Fuels

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Performance of Bimetallic Additives (Fe-Co, Mn-Co, Cu-Co and Zn-Co) Modified Na2S/AC Deoxidizers towards Removal of O2 from Low Concentration Coalbed gas

5

Yuhua Zhanga,b,&, Hongyan Pana,b,&, Qian Lina,b, *, Fuxin Liua,b, Guoxiang Zhanga,b,

6

Pingfeng Hua,b

7

a

8

Guizhou, 550025, PR China. Email: [email protected]

9

b

1 2 3

College of Chemistry and Chemical Engineering, Guizhou University, Guiyang,

Guizhou Key Laboratory for Green Chemical and Clean Energy Technology, Guiyang,

10

Guizhou 550025, China

11

ABSTRACT: In this study, the deoxidizer Na2S was modified by different transition

12

bimetals (Fe-Co, Mn-Co, Cu-Co and Zn-Co) and Fe-Co ratios as auxiliary. The

13

obtained deoxidizers were characterized by XRD, N2 adsorption/desorption, XPS and

14

H2-TPR, and their deoxidizing activity for the removal of O2 from low concentration

15

coalbed gas (CBM) were studied. Results showed that reactive oxygen species (O£-)

16

content in Na2S deoxidizer, as well as unit effective deoxidization capacity （ QO2 ）and

17

conversion rate of ( C Na2S ) all increased after modification with Fe-Co, Mn-Co and

18

Cu-Co. The higher the O£- content in deoxidizers, the higher activity of the deoxidizers

19

for the removal of O2 from CBM was. Fe/Co-Na2S/AC had the highest amount of O£-

20

content, larger pore volume and less impurity species with small crystal size, and this

21

made it the largest QO2 (63.74 mL/g) and C Na2S (87.24%) at 200 oC and 1atm. 1

ACS Paragon Plus Environment

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1

Although the deoxidizer Zn/Co-Na2S/AC modified with Zn-Co had larger O£- content,

2

QO2 and C Na S of this deoxidizer is the lowest due to the presence of more impurities

3

CoS2, CoO, and NaOH with larger crystal size on its surface. These impurities blocked

4

the pores of the deoxidizer Zn/Co-Na2S/AC and inhibited both mass transfer and

5

reaction of reactants through the material, resulting in declined activity. The deoxidizer

6

Fe/Co(0.33/1)-Na2S/AC showed higher deoxidizing activity at Fe-Co ratio was 0.33/1.

7

KEYWORDS: Low concentration oxygenated coalbed gas; Deoxidizers; Na2S;

8

Reactive oxygen species O£-; Transition Bimetals

9

1. Introduction

2

10

Low-concentration coalbed gas (CBM) refers to the unconventional natural gas

11

containing concentrations of CH4 below 30% (by volume fraction) and O2 above

12

14%.1,2 The high O2 concentration could easily lead to explosions during transportation

13

or separation and enrichment processes when the mixture encounters open flame or

14

high-temperature heat source, posing great safety hazard.3 In China, those CBM has

15

not been used forcibly due to the lower concentration of CH4 and higher hazard of

16

explosion; hence most of the mixture is directly emitted to atmosphere. It has been

17

reported that annual release of CBM to atmosphere in China reaches as high as 19

18

billion cubic meters.4 This is equivalent to about 200 million tons of standard coal,

19

causing environmental pollution and energy sources waste. 5 Therefore, full use of 2


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Energy & Fuels

1

low-concentration coal bed gas is necessary, and pre-removal of O2 is important for its

2

effective utilization.

3

Some previous reports indicated that chemical reduction by means of metal

4

sulfides, especially Na2S as deoxidizer, is effective for removing O2 from CBM since it

5

does not deplete CH4 during deoxidation6. The removal of O2 from CBM by Na2S is

6

based

7

2 Na2 S  3O2  2 Na2 SO3 . However, the species of Na2SO4 and Na2SO3 will cover

8

the surface of bulk structure of Na2S due to the low dispersion of Na2S,7 and results in

9

poor deoxidation effect. In order to enhance Na2S dispersion, Na2S was dispersed on

10

the surface of activated carbon (AC), and transition metals (Cu, Fe, Ni, and Co) were

11

used as additives to prepare deoxidizers M-Na2S/AC (M= Cu, Fe, Ni, and Co) in our

12

previous reports. 8 And the results showed that the deoxidizer Na2S/AC modified by

13

Co had the highest deoxidizing activity due to its largest content of oxygen species

14

(O£-), which is generated by transferring electrons form metal additives to oxygen

15

molecules. However, the ability of a single transition metal to lose electron to activate

16

oxygen molecule is limited. Thus, Na2S conversion rate of deoxidizers changed by a

17

single metal promoter is still low.

on

the

following

equations:

Na2 S  2O 2  Na2 SO4

and

18

Other studies have pointed out that electron loss ability of transition metals is

19

closely related to its electron holes number, and the best effect is obtained when the

20

number of electron holes is close to that of electrons transferred by the reactant 3


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1

molecule. 9,10 The thermodynamic analyses of the reaction between Na2S and O2

2

revealed the ground state oxygen molecules O2 has two unpaired electrons,11 so that the

3

number of electrons required to transfer Na2S and O2 is 2. However, the number of

4

electron holes in single transition metal is fixed and often far from 2, limiting the effect

5

of Na2S conversion rate. Therefore, regulating the number of electron holes in the

6

deoxidizer, in such a way to be close to that of the electron transferred by the reactant

7

molecules, is the key for improving the deoxidization activity of the deoxidizer.

8

Some studies suggested that double transition metal catalysts are able to regulate

9

the number of electron holes to yield significantly better catalytic activities than single

10

transition metals. For instance, Huo et al.12 studied the catalytic performances of Ni-Co

11

bimetallic catalysts during F-T Fischer-Tropsch reaction synthesis and found that

12

introduction of Co in Ni promoted the dispersion of Ni and improved the stability of the

13

active metal particles. Du et al. 13 noticed that doping small amounts of Fe in CO

14

hydrogenation reaction improved the catalytic activities of obtained Co-based catalysts.

15

Therefore, we attempted in this study to adjust the number of electron holes in the

16

metals using two transition metals, which can supply electrons to activate oxygen

17

molecules to form oxygen species O£-. This process would also increase the O£- content

18

in obtained deoxidizers and improve their deoxidization activities.

19

Among commonly used transition metal ions, Fe2+ is previously shown to possess

20

the strongest electron loss ability with average number of electron holes in Fe-Co

21

bimetal reaching 1.95, close to that of electrons required to react with Na2S and O2. 4


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Energy & Fuels

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Hence, Fe-Co was used here as auxiliary agent, Na2S as active component, and AC as

2

carrier to prepare deoxidizer. Bimetals (Zn-Co, Cu-Co, and Mn-Co) with different

3

electron loss abilities and electron-hole numbers were also tested as additives to

4

prepare deoxidizers. The obtained materials were characterized by XRD, N2

5

adsorption/desorption, XPS and H2-TPR, and the effects of bimetal transition on

6

deoxidation performance of low concentration coalbed gas were examined.

7

2. Experimental

8

2.1 Materials and reagents

9

Table 1 shows the used raw materials and reagents.

10

Table 1. Raw materials and reagents used in experiments. Raw materials and

Specifications

Manufacturer

CBM

16% O2, 64% N2, 20% CH4

Guiyang Shenjian Gas Company

N2

99.99%

Guiyang Shenjian Gas Company

AC

-

Henan Liyang Co., Ltd.

Na2S·9H2O

AR

Xilong Chemical Co., Ltd

Cu(NO3)2·3H2O

AR

Fe(NO3)3·9H2O

AR

Mn(NO3)2·6H2O

AR

reagents

Tianjin Hongyan Chemical Reagent Factory Tianjin Komeo Chemical Reagent Co., Ltd. Tianjin Zhiyuan Chemical Reagent Co., Ltd. Guangdong Chemical Reagent Engineering

Co(NO3)2·6H2O

AR

Technology Research and Development Center

Zn(NO3)2·6H2O

AR

NaOH

AR

Tianjin Komeo Chemical Reagent Co., Ltd. Tianjin Komeo Chemical Reagent Co., 5


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Ltd. HCl

1

AR

Chongqing Chuandong Chemical Co., Ltd.

2.2. Deoxidizers Preparation

2

Processing of AC. activated carbon (AC) with 20-60 mesh was screened by

3

dilute hydrochloric acid pickling followed by water washing until reaching pH=7 and

4

drying at 80 °C for overnight.

5

Preparation of deoxidizer N-Na2S/AC. 40 mL mixed solution of Co(NO3)2 and

6

X(NO3)2 (X = Fe, Mn, Cu and Zn) (X/Co=0.33/1, wt%=2%), were added into 50g AC.

7

The impregnation solutions were stirred for 5 h at ordinary temperature, and then dried

8

at 105 °C. Then, these obtained materials were joined into 30 mL Na2S solution (2.78

9

mol/L) at room temperature, respectively, shaken for 1h and transferred to a tube

10

furnace. After drying at 100 °C for 2 h in nitrogen atmosphere, the deoxidizers

11

N-Na2S/AC (N= Fe-Co, Mn-Co, Cu-Co and Zn-Co) with total metal content of about 2%

12

and Na2S content of about 13% were obtained.

13

Preparation of deoxidizer Fe/Co(X)-Na2S/AC. The preparation steps of the

14

deoxidizers Fe/Co(X)-Na2S/AC with different Fe-Co ratio is the same as above, where

15

X represents the Fe-Co ratio. Auxiliary agent Fe-Co ratio in the deoxidizers is 0.11/1,

16

0.33/1, 1/1 and 3/1 respectively.

17

2.3. Characterization

18

The BET surface areas and NLDFT pore size distribution of the deoxidizers were 6


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Energy & Fuels

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determined at 77 K using ASAP2020M analyzer (Micromeritics, USA) and N2 as

2

adsorbate.

3

XRD analysis: X-ray diffraction (XRD, Shimadzu XRD6000 X-ray diffractometer)

4

was used for phase analysis of all specimens under the conditions of CuKα

5

(λ=1.5415Å), voltage of 40kV, current of 100mA, scanning speed of 5°/min, and

6

scanning range of 5°~80°.

7

XPS analysis: X-ray photoelectron spectroscopy (XPS, ESCLAB 250) was

8

employed to gain further understanding of the surface composition of the specimens.

9

XPS was performed using radiation source Kα (15KV, 150W) of Al at binding energy

10

of C (1s) (284.8ev) as reference for correction. The valence band spectra of the

11

elements present on each deoxidizer surface were then compared. Each spectrum was

12

subjected to peak-fitting processing and the data were analyzed.

13

H2-TPR analysis: The temperature-programmed reduction of H2 (H2-TPR) of all

14

specimens were determined by means of Auto Chem II 2920 chemical adsorption

15

instrument (Micromeritics, USA). 500 mg specimen was packed in a quartz tube and

16

passed through 30 mL/min 5 vol% H2/Ar2, and then the temperature of fixed-bed

17

packed with deoxidizer was programmed to 800 °C at heating rate of 10 °C/min.

18

ICP analysis: ICP characterization of the deoxidizers was tested on an IRIS

19

Intrepid II XSP (Thermo Fisher, USA). Working parameters: RF power: 1150 W,

20

nebulizer flow: 26.0 psi and auxiliary gas: 1.0 lpm. 7


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1

Page 8 of 29

2.4 Activity Test

2

Details of the low-concentration coalbed gas deoxidation process has been

3

reported in our previous reports.8 Firstly, 100 g sample M/Co-Na2S/AC was placed in

4

the centre of a fixed bed reactor packed with quartz wool, and then warmed to 100 °C

5

and retained for 2 h at a flow rate of 100 mL/min N2. After that, the temperature was

6

promoted to 200 °C at 10 °C/min, and at this time, nitrogen atmosphere was switched

7

to 100 mL/min low concentration coalbed gas (CH4:20%, N2:64%, O2:16%, Vol%).

8

The exit gas from the reactor was firstly desulphurized by absorption liquid, then dried

9

by silica gel, and finally analyzed by thermal conductivity detector of gas

10 11 12 13

14

chromatograph (GC9560, Shanghai Huaai Chromatograph, China). The unit effective deoxidation amount（ QO2 ） and conversion rate of Na2S ( C Na2S ) of the deoxidizer were calculated by Eq. (1) and (2), respectively:

QO 

（ c0  ct）  Q  t

2

C Na S  2

m

(1)

（ c0  ct）  Q  t

V

(2)

15

where QO2 represents the effective deoxidation unit (mL/g), C Na2S is Na2S

16

conversion rate (%), C0 is O2 concentration of inlet CBM (vol.%), Ct is O2

17

concentration of effluent gas (vol.%), Q is coalbed gas flow rate (mL/min), t is time

18

when O2 is Ct = 0.001 in the effluent gas (min), m is deoxidizer mass (g), and V is the

19

theoretical amount of O2 consumed by the complete conversion of S2-to SO42- (mL). 8


Page 9 of 29

1

3. Results and discussion

2

3.1 Deoxidization performance of N-Na2S/AC modified with different of bimetal

3

3.1.1 XRD analysis

C



 

  

20

CoS2  NaOH  AC



Na2S/AC



  





Zn/Co-Na2S/AC



 





Cu/Co-Na2S/AC









Mn/Co-Na2S/AC









Fe/Co-Na2S/AC



10



CoO





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

30

4

40 50 2()

60

70

80

5

Figure 1. XRD spectra of N-Na2S/AC deoxidizers obtained using N = Zn-Co, Cu-Co, Mn-Co and

6

Fe-Co.

7

Figure 1 shows XRD spectra of N-Na2S/AC before and after modification with

8

Zn-Co, Cu-Co, Mn-Co and Fe-Co. Only the diffraction peaks of C is detected in AC

9

and Na2S/AC, indicating uniform distribution of Na2S on AC . 14 However, when

10

Zn-Co, Cu-Co, Mn-Co and Fe-Co are used to modify the parent deoxidizer Na2S/AC,

11

the diffraction peaks of CoS2 at 2  of 32.53°and 36.50°(JCPDS03-0772) can be

12

observed on all the modified deoxidizers.

13 14

In addition to CoS2 diffraction peaks, the diffraction peaks of CoO (JCPDS75-0419) at 2  around 34.10° and 57.22°, as well as NaOH at 38.22°

9


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Page 10 of 29

1

(JCPDS35-1009) can be observed on Zn/Co-Na2S/AC and Cu/Co-Na2S/AC.

2

Mn/Co-Na2S/AC also reveals that NaOH diffraction peak is observed at 38.22°. That is

3

to say, CoS2, CoO and NaOH are all present in the deoxidizers Zn/Co-Na2S/AC and

4

Cu/Co-Na2S/AC. CoS2 and NaOH are present in Mn/Co-Na2S/AC, but only CoS2 is

5

present in Fe/Co-Na2S/AC.

6

Comparison the diffraction peak intensities of the modified deoxidizers, it can be

7

seen that the peaks intensity of CoS2, CoO and NaOH in Zn/Co-Na2S/AC is the

8

strongest,

9

Fe/Co-Na2S/AC. Note that high-intensity diffraction peaks reflect small width of half

10

peak and larger crystal size, prone to more serious crystal agglomeration. Thus, the

11

crystal agglomeration of CoS2, CoO and NaOH in Zn/Co-Na2S/AC is more serious,

12

which provides less active area of component Na2S for the adsorption and reaction O2

13

from CBM and decreases its deoxidizer activity. By contrast, the crystals of CoS2 and

14

NaOH in Mn/Co-Na2S/AC and CoS2 in Fe/Co-Na2S/AC appear smaller in size with

15

better dispersion, which provides more active area of component Na2S and enhancing

16

the deoxidizer activity.

17

3.1.2 N2 adsorption/desorption isotherms and ICP characterization

followed

by

Cu/Co-Na2S/AC,

and

then

10


Mn/Co-Na2S/AC

and

Page 11 of 29

(a)

3.0 3

6

4

Na2S/AC Zn/Co-Na2S/AC

2

Cu/Co-Na2S/AC Mn/Co-Na2S/AC Fe/Co-Na2S/AC

0

Na2S/AC

(b)

Zn/Co-Na2S/AC 2.5

Cu/Co-Na2S/AC Mn/Co-Na2S/AC

2.0

Fe/Co-Na2S/AC

1.5 1.0 0.5 0.0

0.0

1

Differentail Pore Volme (cm /g)

8

Quantity Adsorbed (mmol/g)

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

0.2

0.4 0.6 0.8 Relative Pressure (p/p0)

1.0

1

Pore Width (nm)

10

2

Figure 2. N2 adsorption/desorption isotherms of N-Na2S/AC（N = Zn-Co, Cu-Co, Mn-Co and

3

Fe-Co) (a), and their pre size distribution (b).

4

Figure 2 depicts the N2 adsorption/desorption isotherms and NLDFT pore size

5

distributions of the deoxidizers N-Na2S/AC. As shown in Figure 2a, N2 adsorption

6

amount of each deoxidizer increased rapidly at p/p0 < 0.05, typical of micropore filling.

7

With the increasing of relative pressure, N2 adsorption amount in each deoxidizer

8

remained basically unchanged, indicating absence of large mesopores in the

9

deoxidizers. 15 However, compared to Na2S/AC, N2 adsorption capacity of the

10

modified deoxidizers reduces. Among them, N2 adsorption capacity of the deoxidizer

11

Zn/Co-Na2S/AC declines seriously. The NLDFT pore size distributions of the

12

deoxidizers in Figure 2b further confirms that the deoxidizers are predominantly in the

13

form of microporous.

14

The micropore volume (Vmic) and BET surface area (SBET) of the deoxidizers are

15

calculated based on data of Figure 2a and the data are present in Table 2. The SBET and

16

Vmic of the modified deoxidizers are lower than that of Na2S/AC, especially for the 11


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Page 12 of 29

1

sample Zn/Co-Na2S/AC, who shows the lowest SBET and Vmic, estimated to about 26.45%

2

and 26.98% lower than those of Na2S/AC, respectively. This is due to the presence of

3

impurities CoS2, CoO and NaOH with larger crystal size in the deoxidizer

4

Zn/Co-Na2S/AC (Figure 1), which blocks the pores of the deoxidizer and minimizes

5

their SBET and Vmic. However, the SBET and Vmic of the deoxidizer Fe/Co-Na2S/AC are

6

slightly lower than those of Na2S/AC, attributed to the uniform dispersion of the

7

additive Fe-Co and active component Na2S on the carrier AC (Figure 1).

8

The ICP of bimetals (Fe-Co, Mn-Co, Cu-Co and Zn-Co) and Na2S in the

9

deoxidizers is also shown in Table 2. The data indicates that the loading amounts of

10

Na2S and bimetallic on the deoxidizers are about 13% and 2% respectively.

11

Table 2. BET specific surface area, micropore volume and element loading of N-Na2S/AC

12

deoxidizers

Samples

Zn/Co-

Zn/Cu-

Zn/Mn-

Zn/Fe-

Na2S/AC

Na2S/AC

Na2S/AC

Na2S/AC

Na2S/AC

SBET/m2·g-1

620.1

456.1

534.0

571.7

606.8

Vmic/cm3·g-1

0.245

0.179

0.211

0.215

0.240

Na2S loading (wt%)

12.56

12.34

12.50

12.49

12.53

bimetal loading (wt%)

0

1.86

1.83

1.85

1.88

13

SBET: BET specific surface area, Vmic: micropore volume.

14

3.1.3 XPS Analysis 12


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Energy & Fuels

(a) O1s

C=O O-H

C=O O-H 1

2+

Co2p3/2

Co 3+ Co

(c)

(b) O1s

O£NaKLL

Zn/Co-Na2S/AC

Zn/Co-Na2S/AC

AC Cu/Co-Na2S/AC

Cu/Co-Na2S/AC

Mn/Co-Na2S/AC

Mn/Co-Na2S/AC

NaKLL Na2S/AC Fe/Co-Na2S/AC

Fe/Co-Na2S/AC

524

526

528

530

532

1

534

536

538

540

542

524

528

B.E.(eV) (d) Zn2p3/2

Zn/Co-Na2S/AC

Zn

Zn

532 B. E.(eV)

(e) Cu2p3/2

Cu/Co-Na2S/AC

536

(f) Mn2p3/2

Mn/Co-Na2S/AC

Zn

Mn

2+

Cu

2+

774

776

778

2+

Mn

Fe/Co-Na2S/AC

2+

780

Fe

782

784

786

(g) Fe2p3/2

2+

Fe

Mn

Cu

+

Cu

772

B.E.(eV)

2+ 2+

544 770

540

Mn

3+

Fe

3+

2+

2+

Cu

Fe

2

1019

1020

1021

1022

1023

1024

1025

930

932

934

936

938

940

942

944 636

638

B.E.(eV)

B.E.(eV)

640

642

644

646

648

705

710

715

3+

720

725

730

B.E.(eV)

B.E(eV)

3

Figure 3. XPS spectra of carrier AC, deoxidizers Na2S/AC, and N-Na2S/AC (N = Zn-Co, Cu-Co,

4

Mn-Co and Fe-Co). Reactive oxygen O£- species: O-, O2- or O22-.

5

Table 3. XPS parameters of O1s in carrier AC, deoxidizer Na2S/AC and N-Na2S/AC. Samples

Key type

C=O

O-H

O£-

AC

RC/%

65.44

34.56

-

Na2S/AC

RC/%

65.44

34.56

-

Zn/Co-Na2S/AC

RC/%

31.90

54.85

13.25

Cu/Co-Na2S/AC

RC/%

29.7

55.61

14.69

Mn/Co-Na2S/AC

RC/%

50.74

31.92

17.34

Fe/Co-Na2S/AC

RC/%

35.51

41.14

23.35

6

XPS spectra of O1s and metal auxiliaries in each deoxidizer are shown in Figure 3.

7

Only one peak of O1s located around 532.50eV is observed on carrier AC (Figure 3a).

8

Whereas, apart from O1s peak, a new peak of NaKLL appeared at 535.75eV is

9

observed on Na2S/AC, which is produced by the interference of Na particles on O1s 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

peak. Compared to NaKLL peak in Na2S/AC, the NaKLL peak on modified deoxidizers

2

is shifted to higher binding energies. Among them, NaKLL spectrum peak in

3

Fe/Co-Na2S/AC is the largest (540.50 eV). In addition, the NaKLL energy peak in each

4

deoxidizer became gentle after modification with metal promoter, nearly eliminating

5

the NaKLL peak in Zn/Co-Na2S/AC. The change shape and location of NaKLL peak

6

in modified deoxidizers is mainly due to the producing of new species on these

7

deoxidizers.

8

From Figure 3 a and b, it also can be seen that the O1s peaks of carrier AC and

9

parent deoxidizer Na2S/AC can be divided into two peaks, which belongs C=O and

10

O-H. Whereas, the O1s peaks of N-Na2S/AC modified by bimetal can be divided into

11

three peaks, which belongs to C=O, O-H and active oxygen O£- (O-, O2- or O22-). It can

12

be seen that a new species produced in the modified deoxidizers is active oxygen O£-

13

and the reason for this has to do with the outermost layer of each metal and its ions,

14

prone to losing electrons. In turn, lost electrons can be transferred to activate oxygen

15

molecules and become high-energy spin doublet O2-. The obtained O2- is further

16

reduced to singlet state of O22- to form active oxygen O£-.

17

In Figure 3c, Co2p3/2 peak appeared on each deoxidizer after modification with

18

metal promoter but with different binding energies, mainly distributes from 777.05eV

19

to 781.21eV. The Co2p3/2 peak binding energy of Fe/Co-Na2S/AC is the smallest

20

(777.05eV) and that of Zn/Co-Na2S/AC is the largest (781.21eV). Co2p3/2 peaks on all

21

the deoxidizers can be divided into two peaks, corresponding to Co2+ and Co3+. The 14


Page 14 of 29

Page 15 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

peak of Zn2p3/2, Cu2p3/2, Mn2p3/2 and Fe2p3/2 on the modified deoxidizers also can

2

be divided into two peaks, corresponding to Zn and Zn2+, Cu+ and Cu2+, Mn and Mn2+,

3

and Fe3+ and Fe2+, respectively. It is noted that among deoxidizers, Zn, Cu+, Mn, Mn2+

4

and Fe2+ have the ability to lose electrons, especially for Fe2+, it has the strongest

5

electron losing ability. However, Cu2+, Zn2+ and Fe3+ don’t have the ability to lose

6

electron due to their highest oxidation states.

7

Table 3 lists the binding energy of each species and its peak area of the

8

deoxidizers. O£- content of the deoxidizers decreases as follows, Fe/Co-Na2S/AC >

9

Mn/Co-Na2S/AC > Cu/Co-Na2S/AC > Zn/Co-Na2S/AC. It means that O£- content of

10

Fe/Co-Na2S/AC is the largest, whereas, that on the Zn/Co-Na2S/AC is the lowest. The

11

reason for this is that Fe2+ of Fe/Co-Na2S/AC has the strongest ability to lose electrons

12

16

13

Zn/Co-Na2S/AC is the weakest and it has the lowest loss electrons ability. And thus

14

Fe/Co-Na2S/AC has the largest O£- content, but Zn/Co-Na2S/AC has the smallest.

15

Though Cu+ in the deoxidizer Cu/Co-Na2S/AC has loss electrons ability, its outermost

16

electron is fully charged,17 and its electron loss ability is weaker than those of Mn and

17

Mn2+ in Mn/Co-Na2S/AC. Thus, the corresponding deoxidizer showed less active

18

oxygen O£- content than Mn/Co-Na2S/AC.

19

3.1.4 Evaluation of N-Na2S/AC deoxidizer performance

in compared with Cu+, Mn, and Mn2+, but the metal property of Zn of

15


Page 16 of 29

(a)

Na2S/AC

16

Zn/Co-Na2S/AC Cu/Co-Na2S/AC Mn/Co-Na2S/AC

12

Fe/Co-Na2S/AC

8

4

0 0

50

100

150 t (min)

1 (b)

100

25

Na2S/AC

concentration of CH4in outlet stream (%)

concentration of N 2in 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

concentration of O 2in outlet stream (%)

Energy & Fuels

Zn/Co-Na2S/AC Cu/Co-Na2S/AC

95

250

(c)

20

Mn/Co-Na2S/AC

90

200

Fe/Co-Na2S/AC

15

85 80

Na2S/AC

10

75 70 65 0

50

2

100

150

200

250

Zn/Co-Na2S/AC Cu/Co-Na2S/AC Mn/Co-Na2S/AC

5

Fe/Co-Na2S/AC

0 0

50

t (min)

100

150

200

250

t (min)

3

Figure 4. Deoxidation effects of Na2S/AC and N-Na2S/AC deoxidizer (N = Zn-Co, Cu-Co, Mn-Co

4

and Fe-Co).

5

Table 4. Effects of metal additive N on deoxidation performance of Na2S/AC.

QO2

C Na2S

Samples

t (min)

Na2S/AC

137.78

39.02

53.40

Zn/Co-Na2S/AC

104.12

29.49

40.36

Cu/Co-Na2S/AC

145.20

41.12

56.28

Mn/Co-Na2S/AC

171.09

48.45

66.31

Fe/Co-Na2S/AC

225.08

63.74

87.24

(mL/g)

(%)

6

Figure 4 shows the breakthrough curves of O2 (a), N2 (b) and CH4 (c) from CBM

7

of N-Na2S/AC. As shown in Figure 4a, compared to deoxidizer Na2S/AC, the 16


Page 17 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

breakthrough curves of O2 from CBM of the deoxidizers modified with Cu-Co, Mn-Co

2

and Fe-Co shift to right side. It means that the effective deoxidation time and

3

deoxidizing activity can be increased when the deoxidizers modified with Cu-Co,

4

Mn-Co, and Fe-Co. Among these, the deoxidizing time of Fe/Co-Na2S/AC is the

5

longest. However, the deoxidization curve of the deoxidizer Zn/Co-Na2S/AC modified

6

by Zn-Co shifted to the left side, and the effective deoxidization time is the shortest.

7

The data in Figure 4(b-c) shows that N2 concentration rise rapidly from original

8

64% to 100% and then gradually decreases to stabilize at 76.2% after 30 min. CH4

9

concentration rapidly declines from original 20% to about 0% and then gradually

10

increases to stabilize at 23.8% after 30 min. These trends are consistent with our

11

previous report.8 It can be seen that the deoxidizer can effectively remove O2 from

12

CBM and increase the concentration of CH4 in the gas product.

13

The unit effective deoxidation amounts ( QO2 ) and Na2S conversion rates ( C Na2S )

14

of the deoxidizers are calculated using the data of Figure 4a based on Eqs. (1) and (2),

15

and the results are listed in Table 4. The effective deoxidization time (t), QO2 and

16

C Na2S of the deoxidizers are as follows, Fe/Co-Na2S/AC > Mn/Co-Na2S/AC >

17

Cu/Co-Na2S/AC > Na2S/AC > Zn/Co-Na2S/AC.

18

The data in Table 4 shows that in compared with Na2S/AC, when Fe-Co, Mn-Co

19

and Cu-Co are impregnated onto Na2S/AC, QO2 and C Na2S of Fe/Co-Na2S/AC,

20

Mn/Co-Na2S/AC and Cu/Co-Na2S/AC are enhanced. This is because a new species--

21

O£- (O-, O2- or O22-) is produced on these modified deoxidizers, which is generated by 17


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

transferring electrons of metals in deoxidizer to the adsorbed O2 of CBM and

2

converting it to O£-. Since the dissociation activation energy of O=O bond of O£- is

3

lower than that of ground oxygen molecules (O2), which makes O=O bond in O£- easier

4

to break. Therefore, Na2S reacts more readily with O£- than ground O2. And thus

5

increasing the deoxidizing activity of the deoxidizers modified with Fe-Co, Mn-Co and

6

Cu-Co. Among these, the deoxidizer Fe/Co-Na2S/AC shows the largest unit effective

7

deoxidization capacity and Na2S conversion rate of 63.74mL/g and 87.24%

8

respectively. These values are about 63.36% and 33.84% higher than those of Na2S/AC,

9

respectively. This is due to the deoxidizer Fe/Co-Na2S/AC having the largest O£-

10

content. The higher active oxygen content in deoxidizer is, the easier the reaction

11

between Na2S and O2 is. Meanwhile, this deoxidizer also has larger SBET and Vmic, and

12

hihger dispersion of Na2S and agent bimetal, which is beneficial for the adsorption and

13

reaction of O2 in CBM and makes it having the largest deoxidizing activity.

14

However, it worth noting that although Zn-Co modification increases the content

15

of active oxygen O£- on deoxidizer Zn/Co-Na2S/AC, its deoxidizing activity ( QO2 and

16

C Na2S ) is lower than that of the parent deoxidizer Na2S/AC. This is due to large crystal

17

size of impurities, such as CoS2, CoO, and NaOH, exit on this deoxidizer, which blocks

18

the pores of the deoxidizers and reduces their SBET and Vmic. And thus the presence of

19

these impurities in this deoxidizer reduces the adsorption and reaction of O2 in coal bed

20

gas.

21

Due to the highest deoxidation activity of Fe/Co-Na2S/AC deoxidizer, the effect of 18


Page 18 of 29

Page 19 of 29

1

Fe-Co ratio on its performance was studied to further improve its activity.

2

3.2 Effect of Fe/Co ratio on Fe/Co-Na2S/AC deoxidizer

3

3.2.1 N2 adsorption/desorption isotherms 3.0

(a)

(b)

Fe/Co(0.11/1)-Na2S/AC Fe/Co(0.33/1)-Na2S/AC

2.5

3

6

Fe/Co(0.11/1)-Na2S/AC Fe/Co(0.33/1)-Na2S/AC

4

Fe/Co(1/1)-Na2S/AC Fe/Co(3/1)-Na2S/AC

2

0 0.0

4

Differentail Pore Volme (cm /g)

8 Quantity Adsorbed (mmol/g)

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

0.2

0.4 0.6 0.8 Relative Pressure(p/p0)

1.0

Fe/Co(1/1)-Na2S/AC 2.0

Fe/Co(3/1)-Na2S/AC

1.5 1.0 0.5 0.0 1

Pore Width (nm) 10

5

Figure 5. N2 adsorption/desorption isotherms (a) and pore size distribution (b) of modified

6

Fe/Co(X)-Na2S/AC prepared with different Fe-Co ratios.

7

Figure 5a-b depicts the N2 adsorption/desorption isotherms and NLDFT pore size

8

distributions of the deoxidizers Fe/Co(X)-Na2S/AC, which is prepared with different

9

Fe-Co ratios. The data in Figure 5a shows that all the deoxidizers belong to type I

10

isotherm dominated by microporous adsorption. With the increasing of Fe-Co ratio, N2

11

adsorption amount of the deoxidizers remains basically unchanged. Figure 5b shows

12

that the pore size distribution of the deoxidizers is mainly distributed less than 2nm,

13

indicating the deoxidizers is predominantly in the form of microporous. The data

14

indicate that the change in Fe-Co ratio does not affect the pore structure distribution of

15

the deoxidizer Fe/Co(X)-Na2S/AC when the total Fe-Co loading is 2%.

19


Energy & Fuels

1

3.2.2 H2-TPR

455 520

471

H2Consumption (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

655

528

Fe/Co(3/1)-Na2S/AC Fe/Co(1/1)-Na2S/AC 233

Fe/Co(0.33/1)-Na2S/AC

682

422

517 645

528

432

644 640


470

AC 100

200

2

300 400 500 Temperature (℃)

600

700

800

3

Figure 6. H2-TPR profiles of different Fe-Co ratios modified Fe/Co(X)-Na2S/AC (X =0.11/1,

4

0.33/1, 1/1, and 3/1).

5

Figure 6 displays the H2-TPR profiles of the carrier AC and deoxidizers

6

Fe/Co(X)-Na2S/AC. Two reduction peaks can be observed on the carrier AC between

7

400 oC and 800 oC, and its temperature reduction peak occurs at 470 °C and 640 °C

8

respectively, which is assigned to methanation and the reduction of oxygen-containing

9

functional groups of the carrier AC.18

10

Compared to carrier AC, a new reduction peak between 517°C and 528°C is

11

observed on the deoxidizers Fe/Co(X)-Na2S/AC modified by different Fe-Co ratio,

12

corresponding to the reduction of Fe2O3.19 For the deoxidizer Fe/Co(0.33/1)-Na2S/AC,

13

it is note that the reduction temperature of Fe2O3 is the lowest and the reduction peak of

14

Co2O3 at 233°C is also detected .20 The lower the reduction temperature of the metal

15

oxide is, the weaker the interaction between metal oxide and carrier AC is, which 20


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

leads to the easy reduction of Fe3+ to Fe2+, and yields more Fe2+ content on the

2

deoxidizer. The theory of electron-hole states that Fe2+ has the strongest electron-loss

3

ability, and higher active oxygen O£- content are formed by activating oxygen molecule

4

through electron transfer. This resulted in elevated reactive oxygen species O£- content

5

on the deoxidizer, which is verified by Figure 7.

6

3.2.3 XPS analysis C=O O-H O£-

2+

(a) O1s

Fe Fe

3+

Fe

3+

Fe

2+

Fe/Co(1/1)-Na2S/AC

NaKLL

Fe/Co(1/1)-Na2S/AC

3+


NaKLL


(c) Fe2p3/2


Co2p3/2


NaKLL


(b)

Co 3+ Co

Fe/Co(3/1)-Na2S/AC

NaKLL

Fe/Co(3/1)-Na2S/AC

524

526

528

530

532

7

534

536

538

540

542

776

778

780

B.E.(eV)


Fe

(d)

2+

Fe

3+

782

715

9

714

Fe

Fe2p3/2

Fe/Co(1/1)-Na2S/AC

(e) Fe2p3/2

2+

Fe

3+

Fe

720

716

718

Fe/Co(3/1)-Na2S/AC

Fe

Fe 710

712

720

722

724

B.E.(eV)

Fe

3+

3+

Fe

3+

(f) Fe2p3/2

3+

Fe

705

710

3+

Fe

8

708

784

B.E.(eV)

Fe

3+

2+

3+

725

730 708

710

712

714

716

718

720

706

708

710

712

B.E.(eV)

B.E.(eV)

714

716

718

720

722

B.E.(eV)

Figure 7. XPS spectra of O1s, Co2p3/2 and Fe2p3/2 in each of deoxidizer modified by Fe-Co

10

assistant.

11

Table 5. XPS parameters of Fe2p3/2, Co2p3/2, and O1s in each deoxidizer. WFe（%） Samples


W O£-（%）

WCo（%）

Key type

RC/%

Fe2+

Fe3+

Co2+

Co3+

C=O

O-H

O£-

36.12

63.88

35.64

64.36

48.75

38.02

13.13

21


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 22 of 29


RC/%

66.65

33.35

18.66

81.34

35.51

41.14

23.35

Fe/Co(1/1)-Na2S/AC

RC/%

24.57

75.43

14.98

85.02

36.84

51.65

11.51

Fe/Co(3/1)-Na2S/AC

RC/%

9.35

90.65

65.51

34.49

53.48

38.60

7.92

1

Figure 7 displays the XPS profiles of O1s, Co2p3/2 and Fe2p3/2 in the

2

deoxidizers Fe/Co(X)-Na2S/AC. It can be seen that O1s peak can be divided into three

3

peaks, corresponding to C=O, O-H, and active oxygen species O£-. Co2p3/2 and

4

Fe2p3/2 can be divided into two peaks, corresponding to Co2+ and Co3+, and Fe3+ and

5

Fe2+, respectively.

6

Table 5 lists the fitting results of Fe2p3/2, Co2p3/2 and O1s in each deoxidizer.

7

The active oxygen O£- content on each deoxidizer varied in the following order:

8

Fe/Co(0.33/1)-Na2S/AC >

9

Fe/Co(3/1)-Na2S/AC, which is similar to that of Fe2+ content. The deoxidizer

10

Fe/Co(0.33/1)-Na2S/AC with the largest O£- content is mainly due to its larger content

11

of Fe2+. This is because Fe2+ has the strongest loss electrons ability. In other words,

12

elevated Fe2+ content should induce higher reactive oxygen species formed by

13

activating oxygen molecules through electron transfer.

14

3.2.4 Evaluation of Fe/Co(X)-Na2S/AC deoxidizer performance


22


>

Fe/Co(1/1)-Na2S/AC

>

Page 23 of 29

concentration of O2in outlet stream (%)

18

Na2S/AC

(a)

16


14


12


10 8 6 4 2 0 0

50

100

150 t (min)

105 Na2S/AC

(b)

100


95


90


85 80 75 70 65 0

50

100

2

150 t (min)

200

concentration of CH 4in outlet stream (%)

1 concentration of N2in 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

200

250

(c)

25 20 15

Na2S/AC

10

Fe/Co(0.11/1)-Na2S/AC Fe/Co(0.33/1)-Na2S/AC Fe/Co(1/1)-Na2S/AC

5

Fe/Co(3/1)-Na2S/AC

0 0

250

50

100

150 t (min)

200

250

3

Figure 8. Effect of Fe/Co ratio on oxygen penetration time in Fe/Co(X)-Na2S/AC deoxidizer.

4

Reaction conditions: at 1 atmospheric pressure, 100 mL/min CBM (CH4 20%, O2 16% and N2

5

64%, vol%) reacts when temperatures up to 200 °C.

6

Table 6. Influence of Fe-Co ratio on deoxidation of Fe/Co(X)-Na2S/AC deoxidizer. Samples

7

t (min)

QO2

(mL/g)

C Na2S

(%)


205.60

58.22

79.69


225.08

63.74

87.24

Fe/Co(1/1)-Na2S/AC

196.20

55.56

76.05

Fe/Co(3/1)-Na2S/AC

178.70

50.61

69.26

Figure 8 exhibits the relationship between the concentration of O2 (a), N2 (b) and 23


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 24 of 29

1

CH4 (c) in tail gas and reaction time using Na2S/AC and Fe/Co(X)-Na2S/AC

2

deoxidizers. Compared to Na2S/AC, the deoxidation curves of all deoxidizers shifted to

3

the right side after modification with additive Fe-Co (Figure (a)). The effective

4

deoxidation time increased and deoxidation activity improved. The deoxidizer

5

containing Fe-Co ratio of 0.33/1 showed the longest effective deoxidation time and

6

highest deoxidation activity.

7

Using the data produced from Eqs. (1) and (2) and Figure 8a, the unit effective

8

deoxidation amounts and Na2S conversion rates of each deoxidizer are calculated and

9

the results are listed in Table 6. The sequence of deoxidation activity of the deoxidizers

10

varied

as

follows:


>


>

11

Fe/Co(1/1)-Na2S/AC > Fe/Co(3/1)-Na2S/AC, which is in consistent with the order of

12

Fe2+ and active oxygen O£- content on the deoxidizers. Fe/Co(0.33/1)-Na2S/AC has the

13

largest unit effective deoxidation amount and Na2S conversion rate, it is 63.74mL/g and

14

87.24%, respectively. This is because the deoxidizer Fe/Co(0.33/1)-Na2S/AC has the

15

highest O£- content deduced by transferring electron from deoxidizers to activate

16

oxygen molecules, resulting in superior deoxidation activity.

17

4. Conclusions

18

(1) Compared to Na2S/AC, the deoxidation activity of N-Na2S/AC improved by

19

modification with Cu-Co, Mn-Co, and Fe-Co due to the producing of active oxygen

24


Page 25 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

O£-. The effective deoxidation unit and Na2S conversion rate of the deoxidizer

2

Fe/Co-Na2S/AC is the highest and its value is 63.74 mL and 87.24%, respectively. This

3

is due to the deoxidizer has the largest content of active oxygen O£- and larger

4

micropore volume, and as well as the higher dispersibility of active component Na2S

5

and auxiliary agent. However, though Zn/Co-Na2S/AC deoxidizer modified by Zn-Co

6

contained certain amounts of active oxygen O£-, the chemical reaction of both additive

7

Zn-Co and active component Na2S on the deoxidizer resulted in the formation of

8

non-catalytic active CoS2, CoO, and NaOH. This weakens the deoxidizing activity of

9

the active component. Besides, the large crystal size of these impurities seriously

10

blocks the pores of the deoxidizer, thereby reducing its surface area and pore volume, as

11

well as inhibiting the mass transfer and reaction of reactants through the channels.

12

(2) The deoxidation activity of Fe/Co-Na2S/AC increases first and then decreases

13

with the increasing of Fe-Co ratio. The highest deoxidizing activity of

14

Fe/Co(0.33/1)-Na2S/AC is obtained at Fe-Co ratio of 0.33/1. The latter is related to the

15

elevated electron-depleting Fe2+ content in the deoxidizer at this ratio, as well as to the

16

strongest ability of activated oxygen molecule to yield superior active oxygen O£-

17

content. Hence, the reaction is easy to carry out, leading to the best deoxidation activity.

18

AUTHOR INFORMATION

19

Corresponding Author

20

*Tel.: +86-851-83604936. Fax: +86-851-3625867. E-mail: [email protected]. 25


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

&

2

authors.

3

Funding

These authors contributed equally to this work and should be considered co-first

4

This work was supported by the Science & Technology Foundation of Guizhou

5

Province (No. (2014)2008), the Scientific and Technological Innovation Talents

6

Team of Guizhou (No. 2018-5607), the Foundation of Guizhou Provincial Ministry of

7

Education (No. (2014)267).and Science and technology project of Guizhou Province

8

(No. (2018)2192).

9

Notes

10 11

The author states that there is no competitive economic interest. References

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Performance of Bimetallic Additives (Fe-Co, Mn-Co, Cu-Co and Zn-Co

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