Chung Lei Zang Parte2

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Studies on the decomposing carbon dioxide into carbonwith oxygen-decient magnetite

II. The effects of properties of magnetite on activity ofdecomposition CO2 and mechanism of the reaction

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, PR China

bDepartment of Chemistry, Jilin University, Changchun 130023, PR China

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

Received 15 January 1999; received in revised form 21 June 1999; accepted 2 July 1999

Abstract

The properties of reduction and adsorption of magnetite were investigated using temperature programming technique and

thermogravimetrical (TG) analysis, and the properties of magnetite were correlated with the activity of decomposing CO 2 into carbon.

The smaller the particle size of magnetite, the larger the surface area; and the lower the activated temperature by H2 reduction, and the

faster the activated speed (the speed of lost oxygen), the greater the amount of adsorption of O 2 and CO2, i.e., the higher the activity ofdecomposing CO2 is. There was specic rapid adsorption of CO2 on Fe3O4- ( > 0), with oxygen deciency and this adsorption

accompanied with decomposing CO2 into carbon. Fe3O4- transfers its electron to the carbon in CO2; therefore, CO2 decomposes into

carbon. Simultaneously, Fe3O4- captures the oxygen in CO2 and converts itself into stoichiometric Fe3O4; so, Fe3O4- is deactivated. But

Fe3O4- could be regenerated by activating Fe3O4 by H2 reduction. The reaction of decomposing CO2 into carbon with magnetite belongs to

quasi-catalytic reactions, and is equal to one-side reaction of the reversible process of the water gas shift reaction (WGS). Decomposition of

CO2 into carbon on oxygen-decient magnetite undergoes via two steps: CO23 CO O2 and CO 3 C O, the former step is the

control step of the reaction speed, and the intermediate product of CO is rapidly converted further into carbon. # 2000 Published by

Elsevier Science S.A. All rights reserved.

Keywords: Decomposition of CO2; Magnetite; Performance of reduction; Absorption properties; Mechanism of reaction; Quasi-catalytic reaction;

Deactivation; Activation by reduction

1. Introduction

Magnetite is not only a very good magnetic material, but

is also applied extensively in the catalytic eld. It is a

catalyst industrially applied in synthetic ammonia, FT

synthesis (i.e., the hydrogenation of CO to hydrocarbon,

which was rst studied by Fischer and Tropsch) [1,2],

dehydrogenation of ethylbenzene, oxydehydrogenation of

butene, and water gas shift reaction (WGS). Recently, it has

been found that oxygen-decient magnetite is a good mate-

rial to convert carbon dioxide into carbon [36], but it is not

clear about the mechanism of decomposing CO2 to carbon

on magnetite, and there have not been reports on the relation

between the surface properties, reduction ability, adsorption

properties of magnetite and its activity of decomposition of

carbon dioxide. It is very important that carbon dioxide is

xed into carbon and indirectly converted to oxygen for

purifying the air of the sealed system and clearing away

carbon dioxide and supplying oxygen. It is also signicant to

nd out the mechanism of decomposing carbon dioxide over

magnetite and to control conditions of the activation of

magnetite by H2 reduction.

The preparation of oxygen-decient magnetite and its

activity of decomposing CO2 into C has been reported in our

previous paper [5]. In the present paper, we discussed

emphatically the properties of magnetite (surface area,

Materials Chemistry and Physics 62 (2000) 5261

*Corresponding author. Tel.: 86-411-469-4447; fax: 86-411-4691570

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

0254-0584/00/$ see front matter # 2000 Published by 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 8 - 6


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ability of reduction and properties of thermal desorption of

adsorbed O2 and CO2) and their effects on the activity of

decomposing CO2 into C. Based on the structure of mag-

netite analysed before and after decomposition of CO2 with

the magnetite, the mechanism of reaction was proposed.The deactivation and regeneration of the magnetite were

investigated; moreover, it was compared to the process of

water gas shift reaction.

2. Experimental

2.1. Measurement of surface area of magnetite

The adsorption isotherm of N2 gas of the sample was

measured by ASAP-2000 automatic physical adsorption

instrument, and the surface area was calculated by BETmethod.

2.2. Measurement of reduction performance of magnetite

0.1 g of the magnetite sample, which was sieved through

the 4060 mesh, was pre-treated in N2 at 623 K for 30 min,

and cooled to room temperature. Then, a mixture gas of H2/

N2 1/10, at the rate of 50 ml/min was introduced. Theexperimental temperature programming reduction (TPR)

was carried out at a heating rate of 10 K/min, up to

1073 K. The amount of expending H2 was measured using

a thermal conductivity detector. The thermogravimetric(TG) analysis used Perkin-Elemer TGA-7 thermoanalyzer.

The programming thermogravimetric analysis of H2 reduc-

tion was carried out in hydrogen gas, with a ow velocity of

20 ml/min, from room temperature to 1073 K, with a heat-

ing rate of 20 K/min, according to the temperature program-

ming. The thermostatic thermogravimetric analysis of H2reduction was carried out in hydrogen gas with a ow

velocity of 20 ml/min at 573 K.

2.3. Measurement of adsorption properties of magnetite

0.5 g of the sample, which were sieved through 4060mesh for the temperature programming desorption of

adsorbed O2 (O2-TPD), was pretreated in He, with the rate

of 40 ml/min at 1073 K for l h, and oxygen was adsorbed at

room temperature, or in the process of temperature dropping

from 573 K to room temperature, and then using He,

exhausted physical adsorption of O2 on the surface of the

sample. Thermal desorption of adsorbed oxygen was carried

out at a heating rate of 20 K/min from room temperature to

1073 K, the concentration of the desorbed gas was mon-itored and recorded by a thermal conductivity detector. The

experimental of the TPD of adsorbed CO2 (CO2-TPD) was

carried out by the same procedure as mentioned above for

O2-TPD.

2.4. The thermogravimetric analysis of decomposing

CO2 with magnetite

0.1 g of the magnetite powder was reduced by H2 at the

rate of 20 ml/min at 573 K for 100 min, and then was blown

by He for 10 min, CO2 ow of 20 ml/min was introduced

and weight gain (TG curves) was recorded using Perkin-Elmer TGA-7 thermal analyser. Other experimentals were

carried out by the same procedure, described in the previous

paper [5].

3. Results and discussion

3.1. Effects of the surface area of magnetite on the activity

of decomposing CO2

Table 1 shows the BET surface area of a series of

magnetite samples and the activity of their decomposingCO2. The surface area of BET increased in the order Fe3O4-

IV < Fe3O4-II < Fe3O4-III < Fe3O4-V, which correlated

inversely with the average particle size of magnetites.

The conversion of CO2 and the amount of deposited carbon

increased with Fe3O4-d ( > 0) reduced 3 h in H2 in the order

Fe3O4-II < Fe3O4-III < Fe3O4-V < Fe3O4-IV. It is obvious

that the activity of decomposing CO2 with Fe3O4-d related

directly to the BET surface area of magnetite, i.e., with

increasing the BET surface area, the conversion of CO2 and

the amount of deposited carbon increased, and the partial

pressure of CO2 and CO decreased except for Fe3O4-IV. The

BET surface area of Fe3O4-IV was the smallest among thefour samples, while Fe3O4-IV exhibited the highest activity

in decomposing CO2 because it contained -Fe and Fe1-XO

(to be equal to % 1 of Fe3O4-d), which have very highactivity of decomposing CO2 [7,8].

Table 1

Particle size and BET surface area of the original magnetites and activitya of decomposing CO2 with magnetites samples for reduction 3 h by H2

Samples Particle

size (nm)

BET surface

area (m2/g)

Conversion

of CO2 (%)

Amount of deposited

carbonb (mg)

Partial pressure

of CO2 (105 Pa)

Partial pressure

of CO (105 Pa)

Fe3O4-II 108.8 18.5 64 131.3 0.36 0.15

Fe3O4-III 28.5 25.4 70 155.3 0.30 0.12Fe3O4-IV 135.0 1.5 100 267.8 0 0

Fe3O4-V 8.5 41.6 85 208.9 0.15 0.07

a Time and temperature of reaction are 573 K and 180 min, respectively.b Deposited carbon on the 20 g of magnetite sample, Fe3O4-IV, including Fe3C.

C.-l. Zhang et al. / Materials Chemistry and Physics 62 (2000) 5261 53


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3.2. The performance of reduction of magnetite and its

effects on decomposing CO2

3.2.1. The properties of reduction of magnetite

TPR spectra of the series of magnetite samples Fe3O4-i

(i IIV) are given in Fig. 1. The strong reduction peak ofTPR appeared at 803, 748, 733, and 693 K, respectively,

in the order of Fe3O4-II, Fe3O4-IV, Fe3O4-III and Fe3O4-V,

and the width of half-peak declined in the same order,

except that Fe3O4-II has a shoulder peak at 673 K, but theirrelative area of peaks were nearly equal (1.90 0.05, seeTable 2). The initial reduction temperature declined in the

order Fe3O4-II, Fe3O4-III, Fe3O4-V, and Fe3O4-IV, and were

615, 605, 590, and 530 K, respectively. The four magnetite

samples obtained after TPR were checked by the XRD

spectra, showed that there were not only diffraction peaks

of the spinel structure of magnetite in samples, but there also

appeared stronger characteristic peaks of -Fe, and the

XRD spectra of Fe3O4-II after the shoulder peak of TPR

at 673 K has only characteristic peaks of the spinel struc-

tural compound. It indicated that the strong reduction peaks

of magnetite in the TPR spectra could be assigned tothe reduction of Fe3O4 to -Fe, and the shoulder peak

of Fe3O4-II was ascribed to g-Fe2O3, reducing to Fe3O4(the reduction peak of Fe1-X O did not appear in Fe3O4-IV,

and was concealed in the reduction peak of Fe3O4).

Figs. 2 and 3 show the TG curves of programming reduc-

tion and thermostatic reduction of magnetites. It is evident

that the initial reduction temperature (Ti), the nal tempera-

ture (Tf), the relative weight loss (w/wo), the speed of

weight loss (vif) and the activation speed (O2) (the speed

of losing crystal oxygen is evaluated by the tangent slope of

TG curve) all declined in the order Fe3O4-II, Fe3O4-IV,

Fe3O4-III, and Fe3O4-V, and it agrees with the results of TPR.

From Tables 13, it is also seen that the parameters ofTPR (reduction temperature and width of half peak) and of

TG (activation temperature) decreases and the speed of

reductionactivation increases with declining particle size

of magnetite and increase in the surface area, except for

Fig. 1. Temperature-programmed reduction spectra of magnetite samples

of Fe3O4-i (i IIV).

Table 2

The TPR results of magnetite samples

Sample Initial temperature

of reduction (K)

Temperature of

TPR peak (K)

Width of

half peak (K)

Relative area

of peak

Assignment

of PTR peak

Fe3O4-II 615 673 50 0.30 g-Fe2O33 Fe3O4803 160 1.95 Fe

3

O43

-Fe

Fe3O4-III 605 733 125 1.85 Fe3O43 -FeFe3O4-IV 530 748 210 1.85 Fe1-XO 3 -Fe

Fe3O43 -FeFe3O4-V 590 693 110 1.90 Fe3O43 -Fe

Fig. 2. The poikilothermic thermogravimetric curves of magnetite

samples of Fe3O4-i (i IIV) under temperature-programmed reductiveconditions.

Fig. 3. The thermostatic thermogravimetric curves of magnetite samples

of Fe3O4-i (i IIV) under reductive conditions at 573 K.

54 C.-l. Zhang et al. / Materials Chemistry and Physics 62 (2000) 5261


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Fe3O4-IV. The difference of the reduction performance of

these magnetites was mainly due to the different particle

size and BET surface area by different preparation methods.

3.2.2. Effects of the reduction performance of magnetite on

CO2 decompositionAs described above (Tables 13), it was known that the

activity of decomposing CO2 (the conversion and the

amount of carbon deposited) increased in the order

Fe3O4-i (i IIV) with the same reduction times, whilethe decrease of the reductionactivation temperature (Ti)

and the increase of the reductionactivation speed (O2)

of magnetites are also in the above order. Therefore, the

more easily the sample of magnetite reduces, the higher is

the CO2 decomposing activity. This is probably due to the

larger amount of decient oxygen and the higher degree of

reduction of magnetite under the same reduction condition.

100 mg of magnetite was reduced by H2 at the rate of20 ml/min at 573 K for 100 min and then blown in He gas

for 10 min; nally, carbon dioxide was introduced at the rate

of 20 ml/min their TG curves are shown in Fig. 4. It can

be seen that there was a high-speed weight gain in all

samples in the initial period of CO2 introduction. Carbon

dioxide decomposed rapidly into carbon because carbon

dioxide was adsorbed in the special sites of oxygen-decient

magnetite [6]. After the initial rapid increase, the increase in

weight gain slowed down. The total amount of weight gain

and the speed of weight gain increased in the order Fe3O4-II


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there were two desorption peaks in all samples in the range

of 298 to 1073 K; the low-temperature desorption peaks

appeared at nearly 400 8 K and the high-temperaturedesorption peak appeared above 1073 K. By comparison,

there were three desorption peaks in the TPD spectra of

adsorption oxygen on magnetites at 573 K (Fig. 6), and the

value of the peaks are 410 8 K, 737 9 K and >1073 K,respectively. It is evident that the two oxygen desorption

peaks at about 400 and 740 K arose under very differentadsorption conditions, while the peak above 1073 K main-

tained nearly the same with different adsorption conditions.

This results indicated that these two desorption peaks, at

about 400 and 740 K, may be assigned to the desorption of

oxygen species (O2 and O) adsorbed by magnetite, but

the desorption peaks at above 1073 K was due to the

`desorption oxygen in crystal grating' [9,10] from Fe3O4.

From Figs. 5 and 6, it is known that the peak temperatures

of the desorption peaks at low temperature are nearly

identical for various samples, and the initial desorption

temperature was about 333 K in the O2-TPD spectra, but

the area of the desorption peaks is greatly different. The areaof desorption peaks at low temperature increased in the

order Fe3O4-II < Fe3O4-III


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increased in the order Fe3O4-IV, Fe3O4-II, Fe3O4-III and

Fe3O4-V. Obviously, the amount of adsorption of CO2 at

298 K (i.e., reversible adsorption amount) increased with

the increase of the BET surface area and the decrease of

particle size of magnetites. This agrees with the result of O2-TPD of magnetite. All of the original magnetites, except

Fe3O4-IV, had no desorption peaks of dissociative adsorp-

tion of CO2, while it appeared over all the samples after

TPR. XRD patterns indicated that there were Fe1-XO and

-Fe in the sample of Fe3O4-IV, and Fe3O4-i (i IIV) afterTPR produced Fe3O4- ( > 0) of oxygen-decient spinel

structure and a little amount of -Fe. Therefore, it is

conrmed that the dissociative adsorption of CO2 is a

characteristic of Fe3O4-, Fe1-XO and -Fe, and CO was

produced after desorption at about 600 K because of the

dissociative adsorption of CO2. Although only carbon mon-

oxide in the desorption product of dissociative adsorption ofCO2 was checked by gas chromatography, elementary

carbon was produced in the process of dissociative adsorp-

tion of CO2. The carbon deposited on the magnetites after

CO2-TPD was analysed by elemental analysis (Table 4), and

the results showed that deposited carbon was not detected on

the original magnetites of Fe3O4-i (i I, III, V), but it wasobserved on the magnetites after TPR and the original

magnetite of Fe3O4-IV. It strongly supported the point that

the dissociative absorption of CO2 was not produced on the

original magnetite (Fe3O4, > 0) and it was mainly

produced on oxygen-decient magnetite (Fe3O4-, ! 0).

Tabata et al. [11] considered that there was special adsorp-tion of CO2 (rapid adsorption) on the oxygen-decient

magnetite (Fe3O4-, > 0) and the adsorption was accom-

panied by production of carbon by CO2 decomposition.

Because the Fe3 and Fe2 in oxygen-decient magnetite is

not distinguished, the electrons could hop between Fe3 and

Fe2 in the spinel structure of magnetite, being equal to the

electrons donated by both Fe3 and Fe2 ions, i.e., there was

oxygen deciency in the neighbourhood of both Fe3 and

Fe2, after carbon dioxide was adsorbed. The electrons of

both Fe3 and Fe2 on the adsorption sites donated easily to

the carbon in the carbon dioxide, so carbon dioxide was

reduced. The oxygen in carbon dioxide, in the form of O2,was transferred into lattice vacancy (oxygen deciency),

and carbon dioxide was decomposed to carbon and carbon

monoxide.

3.3.3. The relation between the activity of CO2decomposition and the adsorption properties

of magnetite

The results of O2-TPD and CO2-TPD on magnetites

indicated that the amount of adsorption of oxygen andthe amount of reversible adsorption of CO2 increased in

the order Fe3O4-IV < Fe3O4-II< Fe3O4-III < Fe3O4-V, but

the amount of dissociative adsorption of CO2 increased

in the order Fe3O4-II < Fe3O4-III < Fe3O4-V < Fe3O4-IV.

From Tables 1 and 4, it is seen that the conversion of

CO2 increased in the order Fe3O4-II < Fe3O4-III < Fe3O4-

V < Fe3O4-IV under the same reduction condition of mag-

netite. It is evident that the activity of magnetite to decom-

posing CO2 into carbon increased with increasing the

amount of dissociative adsorption of CO2, the amount of

reversible adsorption of CO2 and adsorption oxygen (except

of Fe3O4-IV), which reected the effects of the adsorptionproperties of magnetite on the activity of CO2 decomposi-

tion.

3.4. Analysis of structure of magnetites before and after

CO2 decomposition

The Mossbauer spectra for magnetite samples of Fe3O4-II

before and after CO2 decomposition were given in Fig. 9,

there Mossbauer parameters and chemical composition

were listed in Table 5. It is obvious that the Mossbauer

spectra of oxygen-decient magnetite of Fe3O4-( 0.1324), reduced for 5 h at 573 K, and that of originalFe3O4-II were very similar (Fig. 9a and b), and the internal

magnetic eld and the area of peaks were slightly varied; the

internal magnetic eld of A-site and B-site were 492.2 kOe

and 459.2 kOe and shifted to 481.2 kOe and 452.3 kOe,

respectively. The area ratio, SB/SA, of these two group peaks

converted from 1.096 to 2.460, i.e., the peak area of A-site

decreased and the peak area of B-site increased. The SB/

SA 1.096 < 2.0 of the original Fe3O4-II indicated thatthere was a considerable amount of g-Fe2O3 (about

21.5% Fe) in the original Fe3O4-II, a part of Fe3 in the

spinel structure was reduced to Fe2 in the Fe3O3.8676 andthe SB/SA was above 2.0, which is oxygen decient. After

Fe3O3.8676 was reacted with CO2 for 3 h, the Mossbauer

spectrogram of sample did not change greatly (Fig. 9c), but

its internal magnetite eld was larger than that of Fe3O3.8676and smaller than Fe3O4-II (Fe3O4.1086) and the SB/SA was

1.950% 2.0. The structure and phase of Fe3O4-II, whichwas determined by XRD, before and after reduction indi-

cated that they were spinel structure compound yet and

the lattice constants of the original Fe3O4-II, the reduced

Fe3O4-II and the reacted Fe3O4-II were 0.8389, 0.8410 and

0.8397 nm, respectively. It is seen that the oxygen in the

crystal grating of the original Fe3O4-II in is excess because it

contains g-Fe2O3 and its lattice constant is smaller than

0.8397 nm of the stoichiometric Fe3O4 [12], Fe3O3.8676obtained by reducing the original Fe3O4-II for 5 h is oxygen

Table 4

The amount of carbon deposited on magnetites after CO2-TPD, including

the carbon in Fe3C

Samplesa Deposited

carbon (mg)

Samplesb Deposited

carbon (mg)

Fe3O4-II 0 Fe3O4-II 0.27

Fe3O4-III 0 Fe3O4-III 0.85Fe3O4-IV 0.65 Fe3O4-IV 1.25

Fe3O4-V 0 Fe3O4-V 0.98

a Original magnetite.b After TPR in the range of 298673 K.



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decient and its lattice constant is larger than the stoichio-

metric Fe3O4, the lattice constant of Fe3O3.8676 decomposed

CO2 for 3 h was nearly equal to that of Fe3O4, indicating that

Fe3O4- of oxygen-decient magnetite completely con-

verted to stoichiometric Fe3O4 after decomposition ofCO2. After reducing Fe3O4 -II for 20 h, its Mossbauer

spectrogram changed greatly (Fig. 9d), there were three

groups of six-line spectrum of magnetic splitting in the

Mossbauer spectrogram, two groups of six-line spectrum for

internal magnetic eld of 477.8 kOe and 451.0 kOe were

very similar to the original Fe3O4-II, which is due to the A-

site and B-site of the spinel structure, Its internal magnetic

eld is lower than Fe3O3.8676 after reducing for 5 h and its

chemical composition is Fe3O3.8277, and the area ratio of

these two peaks is 2.596. Another group of six lines for

332.4 kOe was same as the characteristic Mossbauer spec-

trogram of-Fe and the amount of-Fe that is also checkedby XRD is 22.8%. For the sample Fe3O3.8277, after reacting

in CO2 for 3 h at 573 K, there appeared four groups of six-

line spectrum in the Mossbauer spectra (Fig. 9e). Except the

three groups of six-line spectrum before reaction, one six-

line spectrum for internal magnetic eld of 209.2 kOe is

assigned to Fe3C, which is determined by XRD [6,7]. Fe3C

is produced by -Fe reacting with CO2 or the deposited

carbon. When the sample further reacted with excess CO2for 10 h, the characteristic peak of-Fe in the Mossbauer

spectrogram disappeared, the peaks of Fe3O4 and Fe3C

existed and the peak area of these species slightly increased,

which indicated that -Fe completely converted to Fe3O4and Fe3C (Fig. 9f and Table 5).

The Mossbauer spectra and corresponding parameters

for magnetite samples of Fe3O4-IV before and after reaction

Fig. 9. Room temperature Mossbauer spectra for magnetite samples of

original Fe3O4-II (a); reduced Fe3O4-II for 5 h by H2 (b); reaction

of sample (b) with CO2 for 3 h (c); reduced Fe3O4-II for 20 h by H2(d); reaction of sample (d) with CO2 for 3 h (e); and with excess CO2for 10 h (f).

Fig. 10. Room temperature Mossbauer spectra for magnetite samples

of original Fe3O4-IV (a); and after reaction of (a) with CO2 for 3 h (b).



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with CO2 were shown in Fig. 10 and Table 5. There

were three groups of six-line spectrum of magnetic splittingand one group of two-line spectrum in the Mossbauer

spectrogram of Fe3O4-IV. It is evident that the three groups

of six-line spectrum with 485.7, 455.5, and 332.6 kOe of

internal magnetic eld should be assigned to A-site and

B-site of Fe3O4 and -Fe. According to the larger IS

(IS 1.32 mm/s) of the two-line spectrum, it should beascribed to the species containing Fe2, and Fe1-XO was

observed in XRD. When the sample reacted with CO2for 3 h without being reduced, the two-line and six-line

spectra with internal magnetic eld 332.6 kOe of Fe3O4-

IV disappeared and one group of six-line spectrum

of 209.8 kOe appeared in the Mossbauer spectrogram. Itindicated that Fe1-XO and -Fe had reacted with CO2 and

converted into Fe3O4 and Fe3C. From Table 5, it also can be

seen that the amount of Fe3C, which was produced in the

reaction of Fe3O4-IV with CO2, was larger than the amount

of -Fe in Fe3O4-IV before reaction, it indicated that the

Fe3C was not produced only from the reaction between

-Fe and CO2. In fact, Fe1-XO undergoes eutectoid

decomposition in the process of the decomposing CO2 with

Fe3O4-IV and decomposes into Fe3O4 and -Fe [13].

Therefore, a part of produced Fe3C may have come from

Fe1-XO.

Above analysis indicated that no matter what, Fe3O4-( > 0) of oxygen-deciency or Fe1-XO or -Fe were able to

decompose CO2 into carbon and converted itself into stoi-

chiometric Fe3O4 (or Fe3C).

3.5. An exploration of mechanism of decomposing CO2

into carbon

3.5.1. Mechanism of CO2 decomposition

The structure analysis of magnetite before and after

CO2 decomposition indicated that the spinel structure of

Fe3O4- ( > 0) after CO2 decomposition was still main-

tained, but the determination of the lattice constant and

chemical composition indicated that Fe3O4- (> 0) seized

the oxygen in the CO2, and converted itself into stoichio-

metric Fe3O4; simultaneously, CO2 was decomposed into

carbon. After Fe1-XO and -Fe react with CO2, they would

be converted into Fe3O4 and Fe3C. Fe3O4- ( > 0), Fe1-XO

and -Fe were all able to decompose CO2 into carbon andO2 in the lattice of Fe3O4, i.e., CO23 C O

2-; therefore,

all of them are in the active phase of CO2 decomposition.

Fe3O4 CO2 3573K

Fe3O4 C b 0

Fe1XO CO2 3573K

Fe3O4 C

Fe1XO 3573K

Fe3O4 -Fe

-Fe CO2 3573 K

Fe3O4 Fe3C

From the studies of CO2 decomposition [5], it was also

discovered that decomposing of CO2 by the above active

phases not only produced carbon, but also produced a little

amount of CO. The smaller the -value of Fe3O4-, the larger

Table 5

Chemical composition and Mossbauer parameter of magnetites before and after decomposing CO2

Sample Phase and composition IS (mm/s) QS (mm/s) H (kOe) Fe (%) SB/SA

Fe3O4-II (original) Fe3O4.1086 A 0.38 0.02 492.2 47.7 1.096

B 0.74 0.05 459.2 52.3

Fe3O4-II (5 ha) Fe3O3.8676 A 0.35 0.00 481.2 28.9 2.460

B 0.61 0.00 452.3 71.1

Fe3O4-II (5 ha, 3 hb) Fe3O4 A 0.37 0.01 490.5 33.9 1.950

B 0.68 0.02 455.3 66.1

Fe3O4-II (20 ha) Fe3O3.8277 A 0.28 0.01 477.8 21.3 2.596

B 0.59 0.00 451.0 55.9

-Fe 0.01 0.00 332.4 22.8

Fe3O4-II (20 ha, 3 hb) Fe3O4 A 0.38 0.00 490.2 27.6 1.978

B 0.69 0.01 456.0 54.6-Fe 0.02 0.01 331.5 8.5Fe3C 0.29 0.04 209.2 9.3

Fe3O4-II (20 ha, 10 hb) Fe3O4 A 0.39 0.02 491.3 28.9 1.983

B 0.70 0.01 455.8 57.3

Fe3C 0.26 0.26 208.8 13.8Fe3O4-IV (original) Fe3O4.0014 A 0.42 0.00 485.7 26.5 2.064

B 0.85 0.04 455.5 54.7Fe1-XO 1.32 0.21 0.00 17.1

-Fe 0.07 0.05 332.6 1.7Fe3O4-IV (3 h

b) Fe3O4 A 0.41 0.02 488.3 30.4 1.977

B 0.72 0.01 456.2 60.1Fe3C 0.33 0.01 209.8 9.5

a Reduction time.b Reaction time.



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the amount of CO produced. With the -value increased at

higher reaction temperature (673 K), CO was hardly

observed. According to the above results, we supposed that

CO2 decomposes into carbon on the oxygen-decient

Fe3O4-, perhaps via two steps, i.e., CO23 CO O2

and CO 3 C O2 (the reaction 2CO 3 C CO2 couldhardly occur). Since the disappearance of CO always takes

precedence over disappearance of CO2 on Fe3O4- with

larger -value, i.e., the speed of CO conversion into carbon

is faster than the speed of CO2 conversion, it is evident that

the step of CO23 CO O2 could be the control step of

the reaction speed.

3.5.2. Deactivation and regeneration of decomposing

CO2 over magnetite

It can be seen from Table 4 that the stoichiometric oroxygen-excess magnetite (Fe3O4, ! 0) is inactive toCO2 decomposition. After reacting with CO2, Fe3O4-( > 0) converted to stoichiometric Fe3O4 and thus became

deactivated. The activity of the deactivated Fe3O4 can be

recovered by H2 reduction, i.e., the magnetite after reaction

can be regenerated by H2 and be cyclically used; the active

site of magnetite covered by depositing carbon (about 15%)

would be reactivated. There are two methods to remove

deposited carbon: chemical method rapidly reacting with

H2O above 773 K, i.e., C H2O 3 CO H2; physicalmethod ultrasonic oscillation and centrifugalization of

the suspended solution of magnetite with deposited carbonand then dumped in the liquid. The above-described deac-

tivation of magnetite could be regenerated, and is called

temporary deactivation. If a part of magnetite was reduced

by H2 into -Fe or original magnetite prepared containing

-Fe or Fe1-XO (such as Fe3O4-IV), Fe3C would be pro-

duced after reaction, and Fe3C cannot recover to Fe3O4-( > 0) and thus become deactivated permanently.

If the preparation conditions of the magnetite and the

activation by H2 reduction was controlled not to produce -

Fe or Fe1-XO, the magnetite would not be permanently

deactivated and would be able to regenerate and recover

the reactivity of decomposing CO2 into carbon.

3.5.3. Analysis of the process of decomposing CO2with magnetite

From the comprehensive results of reductionactivation

of magnetite, decomposition of CO2, deactivation and

regeneration of magnetite, it was seen that the total process

of magnetite decomposing CO2 can be divided into two sub-

processes (1) and (2):

Fe3O4 H2 3573K

Fe3O4 H2OY b 0 (1)

Fe3O4 CO2 3573K

Fe3O4 C (2)

CO2 H2 3Fe3O4Y 573K

H2O C (3)

It is evident that the total process, Eq. (3), of decomposing

CO2 into C is equal to the reaction between CO2 and H2 that

produced H2O and C on the magnetite Fe3O4 catalyst, but

these two sub-processes of reductionactivation of magne-

tite, Eq. (1), and decomposition reaction of CO2, Eq. (2),belong to stoichiometric reaction. Therefore, the total pro-

cess, Eq. (3), of the reaction belongs to quasi-catalytic

reaction, i.e., the process of decomposing CO2 into C is

between stoichiometric reaction and catalytic reaction. The

subsidiary reaction is

Fe3O4 CO2 3573K

Fe3O4 CO (4)

Eqs. (4) and (1) were added to get Eq. (5):

CO2 H2 3Fe3O4Y 573K

H2O CO (5)

It is seen that the subsidiary reaction in the process ofmagnetite decomposing CO2 into C equals the process of

hydrogenation of CO2 catalysed by Fe3O4; it is the rever-

sible process of water gas shift reaction, Eq. (6), on indus-

trialisation [14]. But one subsidiary reaction of reversible

process of water gas shift reaction is to produce deposited

carbon, Eq. (3), which is our purpose.

CO H2O 3Fe3O4Y 573K

CO2 H2 (6)

It is well known that Fe3O4 is a good catalyst for water

gas shift reaction and it is able to efficiently catalyse

both the positive and negative reactions. Therefore, thereaction between CO2 and H2 over Fe3O4 mainly produced

CO and H2O, Eq. (5). Though Fe3O4 is also active for the

reverse process of water gas shift reaction, we have carried

out by dividing it into two steps, (1) and (2), i.e., Fe3O4reduced by H2 prepared active Fe3O4- with oxygen-defi-

ciency and Fe3O4-d decomposed CO2 into carbon. As

studied above, carbon dioxide was efficiently decomposed

into carbon, the efficient decomposition of CO2 to C is due

to the result of dividing the reverse reaction of water gas

shift reaction into two steps of (1) and (2), and in the process

of the reaction, the conversion of CO2 is able to achieve

100%, and a little amount of CO produced was furtherconverted into carbon. This is proved to be a efficient

process.

If the water produced from the process Fe3O4 reduced by

H2 was electrolysed to H2 and O2, then after separating H2,

O2 can be put into the sealed cabin of spaceships to maintain

the life systems in spaceships. The purpose of decomposing

CO2 into C over Fe3O4- is to purify the air in the cabin of

spaceships and to clear away CO2. After decomposing CO2,

Fe3O4- could be converted into Fe3O4 and deactivated. The

H2 released by electrolysing water activated Fe3O4 and

converted to Fe3O4-. Thus, this process can clear away

CO2 and supply O2 in the spaceship. It is evident that the

nal sources of oxygen the oxygen in CO2, and CO2itself was xed to carbon after the above-described circula-

tion.



10/10

4. Conclusions

1. The properties of magnetite signicantly affected the

activity of decomposing CO2

into C. The decomposition

of CO2 increased with increase activity of the BET sur-

face area of magnetite, decreasing the particle size and

increasing the amount of adsorption O2 and CO2. The

more easily magnetite is reduced, the higher the decom-

position of CO2 into carbon under the same reduction

conditions is; the decomposition of CO2 increased with

decreasing reductionactivation temperature and in-

creasing the speed of activation in process of TPR.

2. There was special adsorption of CO2 on Fe3O4- and this

adsorption accompanies the decomposing CO2 into

carbon that belongs to dissociative adsorption of CO2.

But Fe3O4 (! 0) occurred only with reversibleadsorption of CO2 and adsorption state of CO2 ismolecular in nature.

3. Fe3O4- could transfer its electron to the carbon in CO2and CO2 was reduced to elementary carbon; simulta-

neously, the oxygen-deficiency of Fe3O4- could capture

oxygen in CO2 and convert itself to stoichiometric

Fe3O4, i.e., become deactivated, and recovers the

activity of decomposing CO2 through its reduction of

H2. Fe3O4-d, Fe1-XO and -Fe are all able to convert

CO2 into C, but only the structure of Fe3O4- before and

after decomposing CO2 was retained and Fe3O4- was

regenerated, but Fe1-XO and -Fe, after decomposingCO2 would convert into Fe3O4 and Fe3C and were

deactivated permanently.

4. The reaction of decomposing CO2 into C with

magnetite belongs to quasi-catalytic reaction, the total

process of reduction by H2 and decomposition of CO2 is

one-side reaction of reverse process of water gas shift

reaction. The reaction of converting CO2 into C with

oxygen-deficient magnetite is very important to

maintain life systems in spaceships and other sealed

systems.

Acknowledgements

Financial support from the National Natural Science

Foundation of China (NNSFC 29703002) is gratefully

acknowledged.

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