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Studies on the decomposition of carbon dioxide intocarbon with oxygen-decient magnetite

I. Preparation, characterization of magnetite, and itsactivity of decomposing carbon dioxide

Chun-lei Zhanga,b,*, Shuang Lib, Li-jun Wangb, Tong-hao Wub, Shao-yi Pengc

aState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, ChinabDepartment of Chemistry, Jilin University, Changchun 130023, China

cShanxi Institute of Coal Chemistry, Chinese Academy of Science, Taiyuan 030001, China

Received 15 January 1999; received in revised form 24 June 1999; accepted 2 July 1999

Abstract

The magnetite Fe3O4 (0 < 0.5) of different particle sizes were prepared using wet-heating oxidation method, supercritical uid dry

method and ferrous oxalate and ferric oxalate high-temperature decomposition method, and their structure were characterized by XRD,

Mossbauer spectra, IR spectra and STEM. The oxygen-decient magnetite of Fe3O4 (0 < < 1) was prepared using H2 reduction of

Fe3O4 (0 < 0.5) at 573 K and its properties (lattice constant, magnetism, stability, and reduction ability) were studied in detail. The

activity of decomposing CO2 with oxygen-decient magnetite of different sizes were compared and the effects of degree of oxygen-

deciency, reaction time, and temperature on the activity of the decomposing CO2 into carbon with oxygen-decient magnetite wereinvestigated. It is found that oxygen in the CO2 was incorporated in the form of O

2 into oxygen-decient magnetite and CO2 was reduced

to carbon, at the same time Fe3O4 converted into stoichiometric Fe3O4. The smaller the particle size of magnetite is, the larger the

oxygen-decient degree is, the higher the reaction temperature is, and the higher the activity of decomposing CO 2 is. # 2000 Elsevier

Science S.A. All rights reserved.

Keywords: Decomposition of carbon dioxide; Magnetite; Oxygen-deciency; Carbon; Mossbauer spectra; Stability; Reduction

1. Introduction

The system of guarantee life is an important part of space

shuttle spaceport and a major content of Life-sustaining

system is how to condense and decompose carbon dioxide. Itis important to clear away carbon dioxide and supply oxygen

as soon as possible for living condition in space capsule and

other sealed and discirculation air environmental system.

Carbon dioxide of living discharge waste gas not only affects

the quality of atmosphere, but also leads to `green house

effect' which is warming the globe and affects natural

environment of man-living with high developing modern

industrialization. Now some developed countries have put

giant fund into studies on the xation of carbon dioxide [1]

to solve the problem of spaceship, charge of warming the

globe, and crisis of energy source.

The xation of carbon dioxide has been extensively

studied [210], but there are only a few reports concerning

the complete decomposition of carbon dioxide into carbon

by ferrites [510]. Recently, we reported the results which

indicate that decomposing efciently carbon dioxide intocarbon (nearly 100%) could be done with magnetite reduced

by H2 which was prepared by wet-heating oxidation method

[11] and ferrous oxalate decomposition method [12], the

spinel structure of magnetite is kept before and after reaction

so that it could easily be regenerated. Reid et al. [12]

studied the Bosch reaction with metal iron in which the

conversion of CO2 is below 30% at above 800 K and a lot of

CO and CH4 of by products were formed, after reaction the

iron changed to be oxides and carbides and couldn't be

regenerated. Therefore, decomposing CO2 with oxygen-

decient magnetite is better than Bosch method [5,11].

The magnetite of different particle sizes were prepared by

four methods in this paper. Based on the reduction activation

and structural characterization of these magnetites, the

activity of decomposing CO2 over oxygen-decient magne-

Materials Chemistry and Physics 62 (2000) 4451

*Corresponding author. Tel.: 86-411-469-4447; fax: 86-411-469-

1570

E-mail address: [email protected] (C.-l. Zhang)

0254-0584/00/$ see front matter # 2000 Elsevier Science S.A. All rights reserved.

P I I : S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 1 6 9 - 8


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tite was studied and correlated with the degree of oxygen-

deciency and the size of magnetite.

2. Experimental

2.1. Preparation of magnetite

According to the wet-heating oxidation method [13,14]

the suspensions of Fe(OH)2 with 2NaOH/FeSO4 1.0 (mol

rate) was oxidized by air ow of 200 ml/min for 20 h at

358 K to obtain Fe3O4-II sample. Aquogel of Fe(OH)3 was

prepared with Fe(NO3)3 solution by adding excess aqueous

ammonia at room-temperature. After ltration, Fe(OH)3was washed by anhydrous ethanol several times which

become ethanol-gel, which was put into a high pressure

chamber by adding anhydrous-ethanol in the chamber. Thechamber was heated at the rate of 80 K/h until the tem-

perature is above supercritical temperature (tc 516 K),

and was kept at 533 K for 30 min. The ethanol was slowly

released and then blown in N2 atmosphere until room-

temperature. Finally, Fe3O4-III was obtained by the super-

critical uid dry method [1517]. Ferrous oxalate and ferric

oxalate were put into quartz tube, respectively, after the

samples were dried in the He gas stream at 473 K, they were

heated rapidly to 923 K and kept at 923 K for 10 h. Products

were cooled to the room-temperature in the He gas to obtain

Fe3O4-IV and Fe3O4-V, respectively.

Twenty grams of above-mentioned magnetite sampleswere reduced by H2 at a rate of 40 ml/min at 573 K for

different time, so the magnetites with different oxygen-

decient degree were prepared.

2.2. Characterization of magnetite and determination of

oxygen-deficient degree

The analysis of X-ray diffraction (XRD) was determined

by Rigaku P/MAX-IIIA type rotary X-ray diffractometry

with Cu K radiation, whose scanning speed of phase

analyzed is 48/min; lattice constant was determined by

using 0.1258/min scanning speed, collecting all X-ray dif-fraction peaks within 20708 according to internal standard

of Si, and calculated with least square method. The average

particle size was measured by small-angle X-ray scattering

method. The shape and particle size of the crystal were

recorded by STEM (Hitachi H-8100). The Mossbauer spec-

tra were recorded by Oxford type accelerating Mobssbauer

spectrometer with a 57Co source at room temperature. The

spectra were calibrated by -Fe and calculated by a com-

puter curve tting method to pure Lorentzian spapes. The

IR-spectra was determined by Nicolet 5PC-IR optical spec-

trometer. Self-supporting wafers were pressed using a mix-ture of samples and CsI.

The amount of Fe2 and total Fe of magnetite which was

dissolved with a concentrated hydrochloric acid were mea-

sured by redox titration with solution of K2Cr2O7 and

SnCl2K2Cr2O7, respectively, using diphenylamine-4-sulfo-

nic acid sodium salt as the indicator. The chemical compo-

sition and the oxygen-decient degree of magnetite were

obtained according to the ratio of Fe2/Fetot [11]. When

magnetite samples contained Fe1xO and/or -Fe, the

amount of Fe2 which is equal to the amount of Fe 1xO

and/or -Fe in sample measuring with Mossbauer spectra,

were deducted from the amount of Fe2 and total Feobtaining by chemical analysis, respectively.

2.3. Decomposition reaction of carbon dioxide and

analysis of the products

A closed reaction cell of 0.5 l without temperature and

concentration gradient with 20 g of oxygen-decient mag-

netite was blown by an extremely pure He gas for 10 min

and then was treated in vacuo for 20 min, nally CO2 was

introduced into the reaction cell and the decomposition

reaction was carried out at different temperatures (CO2

was rst introduced into a vessel with the same volumeas the reactor to reach the pressure of 1.013 105 Pa at

room temperature, then press CO2 from the vessel into the

reactor at reaction temperatures). The inner pressure of the

reaction cell was measured with the pressure gauge and the

inner gas species were analyzed by SP-2305 type gas

chromatography. The carbon deposited on the magnetite

was analyzed by PerkinElmer 2400 CHN elementary

analysis instrument.

3. Results and discussion

3.1. Structure characterization of magnetite

The XRD of series of magnetite samples were given in

Fig. 1, their lattice constants and average particle size were

listed in Table 1, it is obvious that characteristic XRD peaks

of the spinel-type compound appeared in the XRD spectra of

Table 1

Particle size, lattice constant, and chemical composition of magnetite samples

Samples Fe3O4-II Fe3O4-III Fe3O4-IV Fe3O4-V

Average particle size (nm) 108.8 28.5 135.0 8.5

Distribution of particle size (nm) 50.0140.8 16.735.4 88.2180.5 5.012.6

Lattice constant (nm) 0.8389 0.8385 0.8396 0.8393

Chemical composition Fe3O4.1086 Fe3O4.1512 Fe3O4.0014 Fe3O4.0302

C.-l. Zhang et al. / Materials Chemistry and Physics 62 (2000) 4451 45


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all these magnetite samples, but the weak peaks correspond-

ing to -Fe and Fe1xO only appeared in Fe3O4-IV. It

indicated that there were pure phases of spinel type com-pounds of those magnetite samples prepared by these three

methods, except Fe3O4-IV containing a little amount of

-Fe and Fe1xO which was prepared by high temperature

decomposing ferrous oxalate. From Fig. 1, it was seen that

the particle size of samples declined in order of Fe3O4-

IV > Fe3O4-II > Fe3O4-III > Fe3O4-V, which are 135.0,

108.8, 28.5, and 8.5 nm, respectively. It is quite evident

that Fe3O4-V are superne particles. Their lattice constants

are 0.8396, 0.8389, 0.8385, and 0.8393 nm, respectively,

which are smaller than the lattice constant of the stoichio-

metric Fe3O4 (0.8397 nm), and it indicated that all of the

samples whose chemical composition is Fe3O4d ( > 0) areoxygen-excess, this may be caused by the oxidation of parts

of small particles of Fe3O4 into g-Fe2O3 in the process of

preparation, and, therefore, became the solid solution of

Fe3O4 (0 < < 0.5) i.e., the oxygen-excess magnetite

[13].

The Mossbauer spectra of magnetite samples and corre-

sponding parameters are given in Fig. 2 and Table 2. It can

be seen that with the decrease of the average particle size of

magnetite, the Mossbauer spectra broadened and HA and HB

of internal magnetic eld decreased (except of Fe3O4-IV),

isomeric shift (IS) decreased and quadruple splitting

moment (QS ) increased because super exchangeable effect

of A-site and B-site in spinel structure decreased [18]. From

Table 2 it was known, the ratio (S SB/SA) of the peak area

corresponding to B-site and A-site is below 2.0 (stoichio-

metric Fe3O4, S 2.0), it was also further proven that the

samples of original magnetite are oxygen-excess [13].

According to equation of (2 S)/(5S 6) of the cation-

decient concentration given by Topsoe [19], the cation-

decient concentration of Fe3O4-i (i IV, II, III, V) are0.104, 7.87, 11.03 and 2.21%, respectively (the calculated

chemical composition for Fe3O4-II: ion-distribution of the

sample is Fe3[Fe31.1575Fe2

0.7639&0.0787]O4, corre-

sponding oxygen-excess type is Fe3O4.1077), corresponding

chemical composition are Fe3O4.0014, Fe3O4.1077, Fe3O4.1527and Fe3O4.0297, which is very approachable to the results of

chemical analysis in Table 1.

The IR-spectrum of magnetites were shown in Fig. 3, two

strong absorbed peaks appeared in these four samples. The

peak near 600 cm1 belonged to tetrahedral elastic vibra-

tion, and the peak around 400 cm1 was ascribed to octa-

hedral elastic and wresting vibration for these four samples

[20]. Moreover, these absorption bands were signicantly

broadened with the decrease of particle size of samples in

order of Fe3O4-i (i IV, II, III, V).

Fig. 1. X-ray diffraction patterns of magnetite samples.

Fig. 2. Room-temperature Mossbauer spectre of magnetite samples.

46 C.-l. Zhang et al. / Materials Chemistry and Physics 62 (2000) 4451


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3.2. The effect of reduction time on oxygen-deficiency

of magnetite and its resulting properties

It is known from Fig. 4 and Table 2 that the Fe3O4-II

reduced by H2 gas for 5 h at 573 K maintained the

spinel-structure, and no other phases appeared, there wasn't

a little amount of -Fe formed until reached 10 h of

reduction time. The longer the reduction time is, the more

amount of-Fe produced, and the amount of produced -Fe

is about 22.8% when reduction time reached 20 h. Similar

changes occurred on other samples with the increase of

reduction time just that on the Fe3O4-II sample discussed

above.

Table 2

Mossbauer parameter of magnetites at room temperature

Samples Crystal

phase

Coordination

centre

IS

(mm/s)

QS

(mm/s)

H

(kOe)

Fe% SB/SA

Fe3O4-IV Fe3O4 A 0.42 0.01 485.7 27.21 1.9835

B 0.85 0.04 455.5 53.97

Fe1xO 1.31 0.20 0.0 17.01

-Fe 0.07 0.25 332.6 1.81

Fe3O4-II Fe3O4 A 0.38 0.02 492.2 47.70 1.0964

B 0.74 0.02 459.2 52.30

Fe3O4-III Fe3O4 A 0.29 0.08 486.8 53.71 0.8619

B 0.68 0.01 454.0 46.29

Fe3O4-V Fe3O4 A 0.28 0.27 459.4 15.04 1.4267

B 0.67 0.0 414.8 21.38

Fe3O4 A 0.18 0.48 0.0 22.30 1.8520

B 0.66 0.38 0.0 41.29

Fe3O4-II (3 h)a Fe3O4 A 0.36 0.02 485.6 31.82 2. 1447

B 0.62 0.0 455.4 68.18

Fe3O4-II (5 h)a Fe3O4 A 0.35 0.0 481.2 28.94 2.4671B 0.61 0.0 452.3 71.26

Fe3O4-II (10 h)a Fe3O4 A 0.32 0.0 478.4 25.18 2.6230

B 0.60 0.01 450.5 66.12

-Fe 0.04 0.01 331.0 8.80

Fe3O4-II (20 h)a Fe3O4 A 0.28 0.01 477.8 21.31 2.6244

B 0.59 0.0 451.0 55.89

-Fe 0.01 0.0 332.4 22.80

a Reduction time.

Fig. 3. IR spectra of magnetite samples.

Fig. 4. Room-temperature Mossbauer spectra for samples reduced Fe3O4-

II for 5 h (a) and 20 h (b) by H2 at 573 K.



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As shown in Tables 2 and 3 that within 5 h of reduction

time (i.e., before a part of Fe3O4 reduced into -Fe), the

lattice constant and -value of oxygen decient degree

increased and the internal magnetic eld of Mossbauer

spectra declined with increasing reduction time. When

reduction time was over 10 h, the spinel structure initially

disintegrated and a part of it decomposed into -Fe. At this

moment the lattice constant, oxygen-decient degree andinternal magnetic eld of Fe3O4 was still kept basically.

Therefore, the change of lattice constant and internal mag-

netic eld of the samples accompany with the change of

chemical composition, i.e., the larger the oxygen-decient

degree is, the bigger the lattice constant is and smaller is the

inner-magnetic-eld.

It is known from Table 4 that the oxygen-decient

magnetite Fe3O4 ( > 0) is stable in the He gas below

623 K, but it is unstable in the He gas at above 673 K and

decomposed to -Fe and Fe3O4. This agrees with the result

of Darken [21]. The oxygen-decient magnetite is unstable

in air at room temperature and it is rapidly oxidized andconverted into the Fe3O4 (> 0) of oxygen-excess mag-

netite.

By comparison of the oxygen-decient magnetite

Fe3O4 ( > 0) with oxygen-excess and stoichiometric

magnetite Fe3O4 (! 0), it can be seen that the oxy-

gen-decient magnetite has relatively larger lattice constant,

smaller internal magnetic eld of Mossbauer spectrum and

higher chemical-potential energy, thus it is active and

unstable and is easily to decompose into Fe3O4 and -Fe.

At room-temperature, Fe3O4 is able to convert O2 gas into

O2 of crystal grating rapidly, which lled in oxygen-

deciency, and appears stronger reducing ability. Similarly,

it is probable that Fe3O4 converts oxygen in gaseous

oxides, such as CO2, into O2 in the crystal grating of

magnetite and reduces non-oxygen element.

3.3. Reactivity of decomposition of CO2 with the oxygen

deficient magnetite

3.3.1. Decomposing activity of CO2 and its relation

of particle size of sample and reduction time

Table 5 gives the activity of CO2 decomposition over

series of oxygen-decient magnetites with different particle

size. With the same reduction time, it is evident that theconversion of CO2 and the amount of carbon deposited on

the magnetite increased in order of Fe3O4-II < Fe3O4-III 0) prepared by wet-

heating oxidation, supercritical uid dry, ferrous and

ferric oxalate high-temperature decomposing methods,

respectively. With particle size decreasing, internal-

magnetic-eld of Mo ssbauer spectra of magnetite

declined, peaks of Mossbauer spectra and IR spectrum

were signicantly broadened.

2. The -value of oxygen-deficient magnetite Fe3O4( > 0) increased with increasing time of H2 reducing

Fe3O4 ( > 0) at 573 K. Fe3O4 is stable in the He

gas below 673 K. But it is unstable in the He gas at

above 673 K or in the air at room-temperature. Thelarger the oxygen-deficient degree is, and the larger the

lattice constant of Fe3O4 is, the smaller the internal

magnetic field of Mossbauer spectra is, the higher

chemical potential energy of Fe3O4 is, and the more

unstable it is, the stronger the ability of its reduction

is.

3. The oxygen-deficient magnetite of Fe3O4 ( > 0) is

able to efficiently seize oxygen in the CO2 so that CO2decomposed to carbon and the Fe3O4 ( > 0) changed

to be stoichiometric Fe3O4. The smaller the particle size

of magnetite is, the higher reaction temperature is, and

the higher decomposing activity of CO2 is. The largerthe oxygen-deficient degree is, the less amount of

intermediate CO is, the faster the speed of CO2 and CO

conversion into carbon are.

Acknowledgements

Financial support from the National Natural Science

Foundation of China (NNSFC 29703002) are gratefully

acknowledged.

References

[1] A. Sacco, R.C. Reid, Carbon 17 (1979) 459.

[2] M.D. Lee, J.F. Lee, C.S. Change, J. Chem. Eng. Japan 23 (1990) 130.

[3] T. Ishihara, T. Fujita, Y. Mizuhara, Chem. Lett. 22 (1991) 2237.

[4] T. Kodama, K. Tominaga, M. Tabata, J. Am. Ceram. Soc. 75 (1992)

1287.

[5] Y. Tamaua, M. Tabata, Nature 346 (1990) 255.

[6] T. Kodama, Y. Wada, T. Yamamoto, M. Tsuji, Y. Tamaura, Mater.

Res. Bull. 30 (1995) 1039.

[7] M. Tabata, K. Akanuma, K. Nishizawa, J. Mater. Sci. 28 (1993)6753.

[8] M. Tabata, K. Akanuma, T. Togawa, M. Tsuji, J. Chem. Soc.,

Faraday Trans. 90 (1994) 1171.

[9] M. Tsuji, T. Togawa, Y. Wada, T. Sano, Y. Tamaura, J. Chem. Soc.,

Faraday Trans. 91 (1995) 1533.

[10] C. Zhang, S. Li, T. Wu, S. Peng, Mater. Chem. Phys. 58 (1999) 139.

[11] C. Zhang, T. Wu, S. Peng, Mater. Chem. Phys. 44 (1996) 194.

[12] C. Zhang, T. Wu, S. Peng, Chinese Sci. Bull. 41 (1996) 744.

[13] C. Zhang, T. Wu, S. Peng, Sci. China Ser., B 39 (1996) 95.

[14] M. Kiyama, Bull. Chem. Soc. Japan 47 (1974) 1646.

[15] J.N. Armor, F.J. Carlson, J. Mater. Sci. 22 (1987) 2549.

[16] L. Feng, S. Chen, S. Peng, Chinese Chem. Lett. 12 (1991) 973.

[17] C. Yuan, X. Yang, C. Zhang, J. Solid State Chem. 121 (1996) 492.

[18] A.E. Berkowitz, W.J. Schuele, J. Appl. Phys. 39 (1968) 1261.

[19] H. Topsoe, J.A. Dumesic, M.J. Boudart, J. Phys. Paris 35C6 (1974)

411.

[20] R. Waldron, Phys. Rev. 99 (1955) 1727.

[21] L. Darken, R.W. Gurry, J. Am. Chem. Soc. 68 (1946) 798.


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