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Complete reduction of carbon dioxide to carbon and indirect conversion to O2 using cation-excess magnetite
WTERIALS CHEMISTRYAND PHYSICS ELSEVIER
Materials Chemistry and Physics 44 (1996) 194-198
Materials
Science Communication
Complete reduction of carbon dioxide to carbon and indirect conversion to 0, using cation-excess magnetite Zhang Chun-lei aa*, Liu Zhi-qiang a, Wu Tong-hao a, Yang Hong-mao 8, Jiang Yu-zi a, Peng Shao-yi b b Shanxi
a Department of Chemistry, Imtitute of Coal Chemistry,
Jilin University, Chinese Academy
Changchun 130023, China of Sciences, Taiywm 030001,
China
Received 26 April 1995;accepted23 June 1995 Abstract
The cation-deficientmagnetite(Fe,-,O,, 1 > 6 > 0) waspreparedat 358K by the air oxidation of Fe(OH), suspensions, and the cation-excessmagnetite (Fe3 +604, 1 > 6 > 0) was obtained by flowing H, gas through magnetite with a cation deficient compositionat 563K. The stability of the magnetiteis examinedfor different temperaturesand atmospheres.The experimental resultsshow that carbon dioxide wasalmostcompletely( 100°/) decomposedinto carbon by active Fe, +JO4 at 563K. The active Fe3+s0, is transformed into the stoichiometricFe,O, during the transfer of O*- to the Fe, +6O.,, and the Fe, f6 O4 can be regeneratedfrom Fe,O, activated by H,, which can be obtained by the electrolysisof Hz0 (produced by the reduction of Hz), The bigger the cation-excessextent of magnetite, the higher the decompositionactivation of COa, the higher the amount of
decomposed CO*, and the higher the amount of indirectly released 0,. Keywords:
Carbondioxidedecomposition; Cation-excess magnetite;Carbon;Oxygen;Fe(OH),
1. Introduction In the space shuttle cabin, O2 is constantly breathed in and CO2 out, which makes the O2 content in ‘the cabin constantly decrease, while the CO, content continuously increases. So it is necessary to clear away the carbdn dioxide in the cabin in order to maintain the astronaut’s life, and the preparation of 0, from CO2 must be studied. The reduction of gaseous oxides such as CO, and H,O is an important concern in industrial processes, pollution control and the life systeti in the space shuttle cabin. In the Bosch reaction [l-4], carbon dioxide. and hydrogen gas were transferred to the metal iron surface at high temperature, but the decomposition efficiency of
* Corresponding author. 0254-0584/96/$15.00 0 1996ElsevierScience S.A. All SSDIO254-0584(95)01652-B
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reserved
carbon dioxide is only lo-30% at 873-1073 K, and the great amounts of byproducts CO (20%) and CH4 (30%) were produced. Copperthwaite et al. [5] have studied the reaction of carbon dioxide on the surface of other metals using X-ray photoelectron spectroscopy (XPS) and found that only 3% carbon dioxide was decomposed after 6 h at 563 K. Recently, Kodama et al. [6j reported that carbon dioxide qn Fe,0 (6 < 1) can be completely decomposed into carbon (nearly lOO%), which provided a new method for erasuring carbon dioxide, but Fe80 was very unsteady and was easy to convert into a-Fe and Fe,O,. The present paper covers the preparation of cationexcess magnetite, and examines the decomposition activity of carbon dioxide on this magnetite. The good result was that more than 98% of carbon dioxide can be converted into carbon. The relationship between the reaction activity and the cation excess is discussed along with the content of magnetite.
Zirang
Chn-lei
et ai. /Materials
Chemistry
and Physics
44 (1996)
195
194-198
2. Experimental
3. Results and discussion
2.1. Preparation of.the magnetite sample
3.1. Analysis of the crystal phase, crystal lattice constant and chemical composition of the magnetite
One litre of distilled water and 1 1 of 0.72 mall-’ FeS04 solution were added to a 3 1 four neck flask, into which N2 was blown at the rate of 200 ml min-’ in order to stir the solution and remove oxygen in the solution. After rapidly raising the temperature to 358 K, 11 of 1.44 mol I-’ NaOH was added. When the temperature became stable at 358 K, 200 ml min-’ of air was introduced to replace NZ, in which air was used both for stirring and oxidizing Fe(OH),. After crystallizing under air oxidation for 10 h, the mixture solution was extracted, and then washed with a HAc-buffer solution, distilled water and acetone in turn. Dried by an Ar flow at 323 K, the cation-deficient magnetite sample was obtained. At 563 K the samples were reduced and activated by using H2 (40 ml mm-‘) for 40, 80, 180 and 300 min, respectively. After cooling with liquid NZ, magnetites of different cation-excess extent were prepared and then preserved in an Ar atmosphere. 2.2. Characterization of magnetite and determinations of cation-excess extent and chemical component
The crystal phase and lattice constant were determined on a Rigaku P/MAX-IIIA type X-ray diffractometer (XRD). Miissbauer spectra of samples were recorded on a IAC-01 type accelerating Mijssbauer spectrometer. The TG curves were obtained on a TGAtype thermogravimetric meter, and based on the weight loss of samples, the cation-excess extents were calculated. Dissolved by using concentrated HCl, the magnetite was titrated by K,Cr,O, and SnCl,-K,Cr,O,, respectively, using diphenylamine-4-sulfonic acid sodium salt as the indicator, in order to determine the Fe2+ content and total Fe content [7], and the chemical component was obtained according to the ratio of Fe2’ to total Fe. 2.3. The decomposition reaction of carbon dioxide
Ten grams of cation-deficient magnetite powder were put into a 0.5 1 gradientless reactor, and then was reduced and activated using H, (40 ml min-‘) for different times after blowing Ar at 563 K for 10 min and then evacuating for 20 min in vacuum. Finally, pure carbon dioxide (P = 1.013 x IO5 Pa) was decomposed in the reactor, where the species of internal gases were detected via gas chromatography, and the pressure of internal gases was determined using the vacuum pressure meter. The carbon deposited on the sample was analyzed on a Perkin-Elmer 2400 CHN type elemental analyzer.
The XRD pattern of the magnetite is shown in Fig 1. The sample, which was prepared by oxidizing Fe(OH), suspensions with air, has XRD peaks similar to those of stoichiometric Fe,O, (it was found to be spine1 structure using XRD) [8] and has no other peaks. After reducing with H2 for 5 h at 563 K (the reduced sample was cooled by using liquid N, and then preserved in the atmosphere of Ar at room temperature). There are no diffraction peaks the same as that of Fig. l(A) for Fe, 0 and metallic Fe in XRD pattern, which shows that the magnetite cannot be reduced to Fe,0 and metallic Fe under this condition. Mtjssbauer spectra show the same results (Table 1, Fig. 2). After fitting Miissbauer spectra of the synthetic sample, we obtained two groups of six-line spectra, which correspond to A-site (tetrahedral coordination) and B-site (octahedral coordination) of the magnetite. When H, was passed through the surface of the magnetite at 563 K, there was a little change in Mossbauer spectra of the sample, indicating that the internal magnetic field decreased slightly. From Table 1, with increasing reduction time of H,, the internal magnetic field constantly decreased. The value of A-site changed from 3.9186 x lo7 Am-’ before reduction to 3.8664 x 10’ A m-l at 5 h and B-site from 3.6542 x 10’ A m-l to 3.6245 x 107Am-‘.
CZ-Fe
A B
A i
L
a-Fe
Jb !.-iIL L
Fig. 1. XRD patterns of the magnetite: (A) Fe,+,O, or Fe,-,O, (6 > 0); (B) Fe, i6 0, (6 > 0) treated in a Ar atmosphereat 673 K.
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Table 1 Miissbauer spectra data of the magnetite for different H, reduction times Reduction time (min) 0
80 180 300
IS (mms-‘)
QS (mm s-l)
Hi x 10’ (A m-l)
Coordination
center
0.3829 0.7446 0.3892 0.7636 0.4086 0.8124 0.4208 0.8504
0.0199 0.0525 0.0192 0.0518 0.0201 0.0513 0.0183 0.0504
3.9168 3.6542 3.9053 3.6513 3.8859 3.6387 3.8664 3.6245
Tetrahedroid Octahedron Tetrahedroid Octahedron Tetrahedroid Octahedron Tetrahedroid Octahedron
(A-site) (B-site) (A-site) (B-site) (A-site) (B-site) (A-site) (B-site)
-10 -8 -6 -4
-2
0
2
4
6
8 10
Velocity (nn/s) Fig. 2. Miissbauer spectra of the magnetite prepared by oxidizing Fe(OH), suspension solution for 10 h.
The weakening of the internal magnetic field of Mtissbauer spectra is because part of Fe3+ in the magnetite is reduced to Fe’+. Thus, we can reduce Fe3+ in the magnetite at 563K by using H, in order to prevent the spine1 structure from being destroyed, which is verified by using the crystal lattice constant and chemical analysis results (Table 2). From Table 2, the crystal lattice constant of the magnetite powder synthesized by oxidizing Fe(OH), using air (particle size50-200 nm) is 0.83886nm, being less than that of stoichiometric Fe,0,(0.83967 nm) [9], and the chemical composition is Fe,,8ss04 (6 = -0.115), where the Table 2 The crystal lattice constant and chemical composition of the magnetite for different H, reduction times Reduction time (min) 0
40 80 180 300 After reaction
Crystal lattice constant
Chemical composition (Fe 3+aw
0.83886 0.83943 0.83984 0.84092 0.84179 0.83960
Fe,,ss50,, 6 = -0.115 Fe,,,950,, 6 = -0.045 Fe,,,,,O,, 6 = 0.023 Fe3.07204, 6 g 0.072 Fe,,,,sO,, 6 = 0.128 Fe,,,,,O,, 46 = -0.009
cation is apparently deficient. The cation decifiency is mainly due to low valence Fe ions (particle size is < 1 pm) being oxidized. The Miissbauer spectra of the sample provide the evidence that the Fe(II) on the B-site in the spine1 structure is partially oxidized-the ratio of Fe(W) on A-site and Fe(II1) and Fe(I1) on B-site corresponding peak area is half the time of that of the stoichiometric magnetite [ 10,111.Similarly, after small particle size ( < 1 pm) Fe,04 is abundantly oxidized, cation-deficient magnetite (Fe, -$ 04, 6 > 0) is certainly produced [lo]. After the magnetite was reduced by Ha, its crystal lattice constant had become bigger, and increased with increasing H2 reduction time (Table 2). At 300 min, the lattice constant had changed from 0.83886 nm before reduction to 0.84179nm, which is bigger than that (0.83967 m-n) of stoichiometric magnetite at 563K. The increase in the lattice constant is accompanied by the change of the chemical composition of magnetite, i.e., Fe2,88504 before the reduction (6 = -0.115), and Fe3,12804(6 = 0.128) at 300 min of the reduction, showing the increase in the content of Fe2* ion. Thus, the change in the lattice constant is of great concern to Fe’+ reduction from Fe3* in spinel, i.e., to the cation-excesscontent of Fe, +JO4 (6 > 0). Due to the presenceof the excesscation in spine1holes after the reduction, the crystal lattice constant increases. The TG results show that the weight is lost in the range of 523K to 673 K, with H20 detected in the tail gas in gas chromatography. The chemical composition calculated by the weight loss of magnetite is very similar to that obtained from chemical analysis. Therefore the weight loss is due to 02- deprivation by Hz. 3.2. The efect of dferent treatment conditions on the lattice structure of the cation-excess magnetite
Only spine1 structure diffraction peaks exist in the XRD pattern, showing that the cation-excessmagnetite is stable at room temperature in Ar atmosphere. The results in Table 3 further demonstrate this conclusion.
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Table 3 Composition of magnetite after H, reduction 300 min in air and Ar gas Time
Chemical composition of magnetite (Fe, + 6 0,)
In As (298 K)
5 min 3h 48 h
Fe,,,,s04, 6 = 0.128 Fe,,,,,O,, 6 = 0.128 Fe,,,,,O,, 6 = 0.126
In air (298 K)
0 min 1 min 10 min 48 h
Fe3,,2804, Fe,,,,O,, Fe,,,,,O,, Fe,,,,,O,,
6 6 6 6
= = = =
0.128 0.022 -0.092 -0.112
6 d
2
1
But at 673 K, both a-Fe and spine1 type compound peaks are present in XRD pattern of the cation-excess magnetite in Ar atmosphere, indicating that it is unstable. Dieckmann [ 121 studied the cation-excess magnetite from 1173 to 1673 K, and found that the cation-excess magnetite exists in the equilibrium state between it and tistite. Darken and Gurry [ 131 studied the cation-excess magnetite at 773 K, and found the stoichiometric magnetite exists in the equilibrium state between it and a-Fe. The cation-excess magnetite (Fe, f6 04, 6 > 0) at room temperature and in an air atmsophere is unstable, it is rapidly oxidized and then becomes the cation-deficient magnetite (Fe, _ s04, 6 > 0). The XRD results indicate that this cation-deficient magnetite is the solid solution of Fe oxidate consisting of Fe,O, and y-Fe,O,, which is in agreement with Tamaura and Tobata’s results [14]. Therefore, we believe that the cation excess magnetite can deprive oxygen of oxidate (for example, COz, etc.) and reduce the corresponding oxidate. 3.3. The relationship between the decomposition activation of casbon dioxide on the cation-excess magnetite and the cation-excess extent (8 value)
Curves A and B in Fig. 3 show the relationship between the time variations and the partial pressures of carbon dioxide and carbon monoxide at 563 K on the magnetite (Fe 3,12804) sample after reduction for 300 min by H,. Apparently, within 30 min of the reaction, the carbon dioxide content decreased rapidly and become only less than 25%. Then the carbon dioxide pressure decreased slowly, after 3 h, as carbon dioxide was completely converted (nearly 100%). On the other hand, as shown in curve B, the carbon monoxide content increased rapidly up to 32% in the initial 40 min of the reaction, then gradually decreased, and finally disappeared after 140 min. During these variations in the carbon dioxide and carbon monoxide contents, the
3
Time(h) Fig. 3. The decrease in Pco2 (A), P,, (B) and inner pressure (C) as a function of time for the reaction between carbon dioxide and Fe,+,O, (6 =0.128).
inner pressure of the reaction cell gradually decreased, which finally became evacuated (shown in curve C in Fig. 3). The sum of the partial pressures of carbon dioxide (curve A) and carbon monoxide (curve B) was nearly equal to the inner pressure (curve C) within 2 h. This suggests that the inner gas mainly consisted of carbon dioxide and carbon monoxide. This is also supported by the gas content analysis using gas chromatography: no other peaks (CH, or C,H,) except carbon dioxide and carbon monoxide were detected. The above analysis results show that carbon dioxide was first decomposed into carbon monoxide on the cation-excess magnetite, and carbon monoxide was further decomposed into C, and eventually the reaction system became a vacuum. From the XRD pattern of the magnetite obtained after the CO, decomposition reaction, we found that the diffraction peaks of the sample were the same as those of the spine1 structure (see Fig. l(A)). The lattice constant and chemical composition are 0.83960 nm and Fe 2,99,O4 (Table 2), respectively, which are very close to the lattice.constant of 0.83967 nm and chemical composition of the stoichiometric Fes04, indicating that the spine1 structure of the magnetite after the reaction was not destroyed, and also showing that the oxygen in the carbon dioxide was transferred in the form of 02- ions into the cation-excess magnetite to form the stoichiometric magnetite. Carbon dioxide was decomposed into C. As determined from the tangent to the curve of carbon dioxide partial pressure as a function of time (Fig. 3A), the carbon dioxide decomposition reaction rate on Fe 3,12804 was found to be 2.93.5 x 10m3 mol min-’ g-l cation-excess magnetite. The results of C elemental analysis show that the C content deposited in the magnetite was nearly equal to the initial amount of carbon dioxide amount in the reaction cell,
Zhang Chun-lei et al. /Materials
Chemistry rind Physics 44 (1996) 194-198
the cation excessextent of magnetite, ‘.hc greater the decomposition activation of carbon dioxide is, and the lower the amount of carbon monoxide obtained in the decomposition process. The carbon deposited on the magnetite can be removed in the form of CH,, obtained by reacting with H2 at 923 K, and then the magnetite (Fe,OJ was reduced by H2 at 563K and regenerated to be the cation-excess magnetite (Fe3+604, 6 > 0). The HZ0 produced in this processwas electrolyzed and released in the form of O2 and H2. H, can then be used in cycles.
d 0
1
2
3
Time(h) Fig. 4. The relationship between the reaction tiime and Pcoz on the magnetite (Fe, + sO,, 6 > 0) obtained for different reduction times: (A) 6 = 0.128; (B) 6 = 0.072; (C) 6 = 0.023.
which supports the conclusion that the carbon dioxide was decomposed into C and that the oxygen in the carbon dioxide was transferred in the form of 02- ions into the cation-excessmagnetite. Fig. 4 shows the relationship between the reaction time and the partial pressure of carbon dioxide. For the same reaction time, the bigger the excessextent of the cation, the higher the 6 value in Fe, +6O4(6 > 0) is, and the lower the partial pressure of carbon dioxide. At 3 h, nearly 100% of the carbon dioxide can be reduced to C on the magnetite (reduced for 300 min) (6 = 0.128), around 90% carbon dioxide on the sample (reduced 180 min) (6 = 0.072) and 75% carbon dioxide on the magnetite (reduced 80 min) (6 = 0.023). So, the bigger
References [l] A. Sacco and R.C. Reid, Carbon, 17 (1979) 459. [2] R.C. Wagner, R. Carrasquillo, J. Edwards and R. Holmes, Proc. 18th Int. Conf. Environmental Syslems, SAE Tech. Pap. Ser 880995, Sot. Automotive Engineers, 1988, pp, 1-9. [3] M.P. Manhing and R.C. Reid, ASME Pap. 75-ENAs-22, Am. Sot. Mech. Engineers, 1975. [4] M. Lee, J. Lee and C. Chang, J. Chem. Eng. Jyn., 23 (1990) 130. [5] R.G. Copperthwaite, P.R. Davis, M.A. Morris, M.W. Roberts and R.A. Ryder, Calal. Lett., 1 (1988) 11. [6] T. Kodama, K. Tominaga, M. Tabata, T. Yoshida and Y. Tamaura, J. Am. Ceratn. SOL, 75 (1992) 1287. [7] CL. Zhang, T.H. Wu, H.M. Yang, Y.Z. Jiang and S.Y. Peng, Chem. J. Chin. Univ., 16 (1995) 955. [8] Joint Committee on Powder Diffraction Standards, Swarthmore, PA, 1989, ICPDS Card 19-629. [9] T. Katsura and Y. Tamaura, Blrll. Chem. Sot. Jpn., 5.2 (1979) [lo] ? Volenik, M. Seberiai and J. Neid, CXC/I. J. Phys., B25(1975) 1063. [ 111 H. Topsoe, J.A. Dumesic and M. Boudart, J. Phys. Paris, C6 (1974) 4ii. [ 121 R. Dieckmann, Ber. Bunsenges. Phys, C/rem., 86 (1982) 112. [ 131 L.S. Darken and R.W. Gurry, J. Am. Chem. SOL, 68 (1945) 798. [14] Y. Tamaura and M. Tobata, Nature, 346 (1990) 255.
Materials Chemistry and Physics 44 (1996) 194-198
Materials
Science Communication
Complete reduction of carbon dioxide to carbon and indirect conversion to 0, using cation-excess magnetite Zhang Chun-lei aa*, Liu Zhi-qiang a, Wu Tong-hao a, Yang Hong-mao 8, Jiang Yu-zi a, Peng Shao-yi b b Shanxi
a Department of Chemistry, Imtitute of Coal Chemistry,
Jilin University, Chinese Academy
Changchun 130023, China of Sciences, Taiywm 030001,
China
Received 26 April 1995;accepted23 June 1995 Abstract
The cation-deficientmagnetite(Fe,-,O,, 1 > 6 > 0) waspreparedat 358K by the air oxidation of Fe(OH), suspensions, and the cation-excessmagnetite (Fe3 +604, 1 > 6 > 0) was obtained by flowing H, gas through magnetite with a cation deficient compositionat 563K. The stability of the magnetiteis examinedfor different temperaturesand atmospheres.The experimental resultsshow that carbon dioxide wasalmostcompletely( 100°/) decomposedinto carbon by active Fe, +JO4 at 563K. The active Fe3+s0, is transformed into the stoichiometricFe,O, during the transfer of O*- to the Fe, +6O.,, and the Fe, f6 O4 can be regeneratedfrom Fe,O, activated by H,, which can be obtained by the electrolysisof Hz0 (produced by the reduction of Hz), The bigger the cation-excessextent of magnetite, the higher the decompositionactivation of COa, the higher the amount of
decomposed CO*, and the higher the amount of indirectly released 0,. Keywords:
Carbondioxidedecomposition; Cation-excess magnetite;Carbon;Oxygen;Fe(OH),
1. Introduction In the space shuttle cabin, O2 is constantly breathed in and CO2 out, which makes the O2 content in ‘the cabin constantly decrease, while the CO, content continuously increases. So it is necessary to clear away the carbdn dioxide in the cabin in order to maintain the astronaut’s life, and the preparation of 0, from CO2 must be studied. The reduction of gaseous oxides such as CO, and H,O is an important concern in industrial processes, pollution control and the life systeti in the space shuttle cabin. In the Bosch reaction [l-4], carbon dioxide. and hydrogen gas were transferred to the metal iron surface at high temperature, but the decomposition efficiency of
* Corresponding author. 0254-0584/96/$15.00 0 1996ElsevierScience S.A. All SSDIO254-0584(95)01652-B
rights
reserved
carbon dioxide is only lo-30% at 873-1073 K, and the great amounts of byproducts CO (20%) and CH4 (30%) were produced. Copperthwaite et al. [5] have studied the reaction of carbon dioxide on the surface of other metals using X-ray photoelectron spectroscopy (XPS) and found that only 3% carbon dioxide was decomposed after 6 h at 563 K. Recently, Kodama et al. [6j reported that carbon dioxide qn Fe,0 (6 < 1) can be completely decomposed into carbon (nearly lOO%), which provided a new method for erasuring carbon dioxide, but Fe80 was very unsteady and was easy to convert into a-Fe and Fe,O,. The present paper covers the preparation of cationexcess magnetite, and examines the decomposition activity of carbon dioxide on this magnetite. The good result was that more than 98% of carbon dioxide can be converted into carbon. The relationship between the reaction activity and the cation excess is discussed along with the content of magnetite.
Zirang
Chn-lei
et ai. /Materials
Chemistry
and Physics
44 (1996)
195
194-198
2. Experimental
3. Results and discussion
2.1. Preparation of.the magnetite sample
3.1. Analysis of the crystal phase, crystal lattice constant and chemical composition of the magnetite
One litre of distilled water and 1 1 of 0.72 mall-’ FeS04 solution were added to a 3 1 four neck flask, into which N2 was blown at the rate of 200 ml min-’ in order to stir the solution and remove oxygen in the solution. After rapidly raising the temperature to 358 K, 11 of 1.44 mol I-’ NaOH was added. When the temperature became stable at 358 K, 200 ml min-’ of air was introduced to replace NZ, in which air was used both for stirring and oxidizing Fe(OH),. After crystallizing under air oxidation for 10 h, the mixture solution was extracted, and then washed with a HAc-buffer solution, distilled water and acetone in turn. Dried by an Ar flow at 323 K, the cation-deficient magnetite sample was obtained. At 563 K the samples were reduced and activated by using H2 (40 ml mm-‘) for 40, 80, 180 and 300 min, respectively. After cooling with liquid NZ, magnetites of different cation-excess extent were prepared and then preserved in an Ar atmosphere. 2.2. Characterization of magnetite and determinations of cation-excess extent and chemical component
The crystal phase and lattice constant were determined on a Rigaku P/MAX-IIIA type X-ray diffractometer (XRD). Miissbauer spectra of samples were recorded on a IAC-01 type accelerating Mijssbauer spectrometer. The TG curves were obtained on a TGAtype thermogravimetric meter, and based on the weight loss of samples, the cation-excess extents were calculated. Dissolved by using concentrated HCl, the magnetite was titrated by K,Cr,O, and SnCl,-K,Cr,O,, respectively, using diphenylamine-4-sulfonic acid sodium salt as the indicator, in order to determine the Fe2+ content and total Fe content [7], and the chemical component was obtained according to the ratio of Fe2’ to total Fe. 2.3. The decomposition reaction of carbon dioxide
Ten grams of cation-deficient magnetite powder were put into a 0.5 1 gradientless reactor, and then was reduced and activated using H, (40 ml min-‘) for different times after blowing Ar at 563 K for 10 min and then evacuating for 20 min in vacuum. Finally, pure carbon dioxide (P = 1.013 x IO5 Pa) was decomposed in the reactor, where the species of internal gases were detected via gas chromatography, and the pressure of internal gases was determined using the vacuum pressure meter. The carbon deposited on the sample was analyzed on a Perkin-Elmer 2400 CHN type elemental analyzer.
The XRD pattern of the magnetite is shown in Fig 1. The sample, which was prepared by oxidizing Fe(OH), suspensions with air, has XRD peaks similar to those of stoichiometric Fe,O, (it was found to be spine1 structure using XRD) [8] and has no other peaks. After reducing with H2 for 5 h at 563 K (the reduced sample was cooled by using liquid N, and then preserved in the atmosphere of Ar at room temperature). There are no diffraction peaks the same as that of Fig. l(A) for Fe, 0 and metallic Fe in XRD pattern, which shows that the magnetite cannot be reduced to Fe,0 and metallic Fe under this condition. Mtjssbauer spectra show the same results (Table 1, Fig. 2). After fitting Miissbauer spectra of the synthetic sample, we obtained two groups of six-line spectra, which correspond to A-site (tetrahedral coordination) and B-site (octahedral coordination) of the magnetite. When H, was passed through the surface of the magnetite at 563 K, there was a little change in Mossbauer spectra of the sample, indicating that the internal magnetic field decreased slightly. From Table 1, with increasing reduction time of H,, the internal magnetic field constantly decreased. The value of A-site changed from 3.9186 x lo7 Am-’ before reduction to 3.8664 x 10’ A m-l at 5 h and B-site from 3.6542 x 10’ A m-l to 3.6245 x 107Am-‘.
CZ-Fe
A B
A i
L
a-Fe
Jb !.-iIL L
Fig. 1. XRD patterns of the magnetite: (A) Fe,+,O, or Fe,-,O, (6 > 0); (B) Fe, i6 0, (6 > 0) treated in a Ar atmosphereat 673 K.
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Chemistry
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44 (1996)
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Table 1 Miissbauer spectra data of the magnetite for different H, reduction times Reduction time (min) 0
80 180 300
IS (mms-‘)
QS (mm s-l)
Hi x 10’ (A m-l)
Coordination
center
0.3829 0.7446 0.3892 0.7636 0.4086 0.8124 0.4208 0.8504
0.0199 0.0525 0.0192 0.0518 0.0201 0.0513 0.0183 0.0504
3.9168 3.6542 3.9053 3.6513 3.8859 3.6387 3.8664 3.6245
Tetrahedroid Octahedron Tetrahedroid Octahedron Tetrahedroid Octahedron Tetrahedroid Octahedron
(A-site) (B-site) (A-site) (B-site) (A-site) (B-site) (A-site) (B-site)
-10 -8 -6 -4
-2
0
2
4
6
8 10
Velocity (nn/s) Fig. 2. Miissbauer spectra of the magnetite prepared by oxidizing Fe(OH), suspension solution for 10 h.
The weakening of the internal magnetic field of Mtissbauer spectra is because part of Fe3+ in the magnetite is reduced to Fe’+. Thus, we can reduce Fe3+ in the magnetite at 563K by using H, in order to prevent the spine1 structure from being destroyed, which is verified by using the crystal lattice constant and chemical analysis results (Table 2). From Table 2, the crystal lattice constant of the magnetite powder synthesized by oxidizing Fe(OH), using air (particle size50-200 nm) is 0.83886nm, being less than that of stoichiometric Fe,0,(0.83967 nm) [9], and the chemical composition is Fe,,8ss04 (6 = -0.115), where the Table 2 The crystal lattice constant and chemical composition of the magnetite for different H, reduction times Reduction time (min) 0
40 80 180 300 After reaction
Crystal lattice constant
Chemical composition (Fe 3+aw
0.83886 0.83943 0.83984 0.84092 0.84179 0.83960
Fe,,ss50,, 6 = -0.115 Fe,,,950,, 6 = -0.045 Fe,,,,,O,, 6 = 0.023 Fe3.07204, 6 g 0.072 Fe,,,,sO,, 6 = 0.128 Fe,,,,,O,, 46 = -0.009
cation is apparently deficient. The cation decifiency is mainly due to low valence Fe ions (particle size is < 1 pm) being oxidized. The Miissbauer spectra of the sample provide the evidence that the Fe(II) on the B-site in the spine1 structure is partially oxidized-the ratio of Fe(W) on A-site and Fe(II1) and Fe(I1) on B-site corresponding peak area is half the time of that of the stoichiometric magnetite [ 10,111.Similarly, after small particle size ( < 1 pm) Fe,04 is abundantly oxidized, cation-deficient magnetite (Fe, -$ 04, 6 > 0) is certainly produced [lo]. After the magnetite was reduced by Ha, its crystal lattice constant had become bigger, and increased with increasing H2 reduction time (Table 2). At 300 min, the lattice constant had changed from 0.83886 nm before reduction to 0.84179nm, which is bigger than that (0.83967 m-n) of stoichiometric magnetite at 563K. The increase in the lattice constant is accompanied by the change of the chemical composition of magnetite, i.e., Fe2,88504 before the reduction (6 = -0.115), and Fe3,12804(6 = 0.128) at 300 min of the reduction, showing the increase in the content of Fe2* ion. Thus, the change in the lattice constant is of great concern to Fe’+ reduction from Fe3* in spinel, i.e., to the cation-excesscontent of Fe, +JO4 (6 > 0). Due to the presenceof the excesscation in spine1holes after the reduction, the crystal lattice constant increases. The TG results show that the weight is lost in the range of 523K to 673 K, with H20 detected in the tail gas in gas chromatography. The chemical composition calculated by the weight loss of magnetite is very similar to that obtained from chemical analysis. Therefore the weight loss is due to 02- deprivation by Hz. 3.2. The efect of dferent treatment conditions on the lattice structure of the cation-excess magnetite
Only spine1 structure diffraction peaks exist in the XRD pattern, showing that the cation-excessmagnetite is stable at room temperature in Ar atmosphere. The results in Table 3 further demonstrate this conclusion.
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Chun-lei
et al. /Materials
Chemistry
and Physics
44 (1996)
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Table 3 Composition of magnetite after H, reduction 300 min in air and Ar gas Time
Chemical composition of magnetite (Fe, + 6 0,)
In As (298 K)
5 min 3h 48 h
Fe,,,,s04, 6 = 0.128 Fe,,,,,O,, 6 = 0.128 Fe,,,,,O,, 6 = 0.126
In air (298 K)
0 min 1 min 10 min 48 h
Fe3,,2804, Fe,,,,O,, Fe,,,,,O,, Fe,,,,,O,,
6 6 6 6
= = = =
0.128 0.022 -0.092 -0.112
6 d
2
1
But at 673 K, both a-Fe and spine1 type compound peaks are present in XRD pattern of the cation-excess magnetite in Ar atmosphere, indicating that it is unstable. Dieckmann [ 121 studied the cation-excess magnetite from 1173 to 1673 K, and found that the cation-excess magnetite exists in the equilibrium state between it and tistite. Darken and Gurry [ 131 studied the cation-excess magnetite at 773 K, and found the stoichiometric magnetite exists in the equilibrium state between it and a-Fe. The cation-excess magnetite (Fe, f6 04, 6 > 0) at room temperature and in an air atmsophere is unstable, it is rapidly oxidized and then becomes the cation-deficient magnetite (Fe, _ s04, 6 > 0). The XRD results indicate that this cation-deficient magnetite is the solid solution of Fe oxidate consisting of Fe,O, and y-Fe,O,, which is in agreement with Tamaura and Tobata’s results [14]. Therefore, we believe that the cation excess magnetite can deprive oxygen of oxidate (for example, COz, etc.) and reduce the corresponding oxidate. 3.3. The relationship between the decomposition activation of casbon dioxide on the cation-excess magnetite and the cation-excess extent (8 value)
Curves A and B in Fig. 3 show the relationship between the time variations and the partial pressures of carbon dioxide and carbon monoxide at 563 K on the magnetite (Fe 3,12804) sample after reduction for 300 min by H,. Apparently, within 30 min of the reaction, the carbon dioxide content decreased rapidly and become only less than 25%. Then the carbon dioxide pressure decreased slowly, after 3 h, as carbon dioxide was completely converted (nearly 100%). On the other hand, as shown in curve B, the carbon monoxide content increased rapidly up to 32% in the initial 40 min of the reaction, then gradually decreased, and finally disappeared after 140 min. During these variations in the carbon dioxide and carbon monoxide contents, the
3
Time(h) Fig. 3. The decrease in Pco2 (A), P,, (B) and inner pressure (C) as a function of time for the reaction between carbon dioxide and Fe,+,O, (6 =0.128).
inner pressure of the reaction cell gradually decreased, which finally became evacuated (shown in curve C in Fig. 3). The sum of the partial pressures of carbon dioxide (curve A) and carbon monoxide (curve B) was nearly equal to the inner pressure (curve C) within 2 h. This suggests that the inner gas mainly consisted of carbon dioxide and carbon monoxide. This is also supported by the gas content analysis using gas chromatography: no other peaks (CH, or C,H,) except carbon dioxide and carbon monoxide were detected. The above analysis results show that carbon dioxide was first decomposed into carbon monoxide on the cation-excess magnetite, and carbon monoxide was further decomposed into C, and eventually the reaction system became a vacuum. From the XRD pattern of the magnetite obtained after the CO, decomposition reaction, we found that the diffraction peaks of the sample were the same as those of the spine1 structure (see Fig. l(A)). The lattice constant and chemical composition are 0.83960 nm and Fe 2,99,O4 (Table 2), respectively, which are very close to the lattice.constant of 0.83967 nm and chemical composition of the stoichiometric Fes04, indicating that the spine1 structure of the magnetite after the reaction was not destroyed, and also showing that the oxygen in the carbon dioxide was transferred in the form of 02- ions into the cation-excess magnetite to form the stoichiometric magnetite. Carbon dioxide was decomposed into C. As determined from the tangent to the curve of carbon dioxide partial pressure as a function of time (Fig. 3A), the carbon dioxide decomposition reaction rate on Fe 3,12804 was found to be 2.93.5 x 10m3 mol min-’ g-l cation-excess magnetite. The results of C elemental analysis show that the C content deposited in the magnetite was nearly equal to the initial amount of carbon dioxide amount in the reaction cell,
Zhang Chun-lei et al. /Materials
Chemistry rind Physics 44 (1996) 194-198
the cation excessextent of magnetite, ‘.hc greater the decomposition activation of carbon dioxide is, and the lower the amount of carbon monoxide obtained in the decomposition process. The carbon deposited on the magnetite can be removed in the form of CH,, obtained by reacting with H2 at 923 K, and then the magnetite (Fe,OJ was reduced by H2 at 563K and regenerated to be the cation-excess magnetite (Fe3+604, 6 > 0). The HZ0 produced in this processwas electrolyzed and released in the form of O2 and H2. H, can then be used in cycles.
d 0
1
2
3
Time(h) Fig. 4. The relationship between the reaction tiime and Pcoz on the magnetite (Fe, + sO,, 6 > 0) obtained for different reduction times: (A) 6 = 0.128; (B) 6 = 0.072; (C) 6 = 0.023.
which supports the conclusion that the carbon dioxide was decomposed into C and that the oxygen in the carbon dioxide was transferred in the form of 02- ions into the cation-excessmagnetite. Fig. 4 shows the relationship between the reaction time and the partial pressure of carbon dioxide. For the same reaction time, the bigger the excessextent of the cation, the higher the 6 value in Fe, +6O4(6 > 0) is, and the lower the partial pressure of carbon dioxide. At 3 h, nearly 100% of the carbon dioxide can be reduced to C on the magnetite (reduced for 300 min) (6 = 0.128), around 90% carbon dioxide on the sample (reduced 180 min) (6 = 0.072) and 75% carbon dioxide on the magnetite (reduced 80 min) (6 = 0.023). So, the bigger
References [l] A. Sacco and R.C. Reid, Carbon, 17 (1979) 459. [2] R.C. Wagner, R. Carrasquillo, J. Edwards and R. Holmes, Proc. 18th Int. Conf. Environmental Syslems, SAE Tech. Pap. Ser 880995, Sot. Automotive Engineers, 1988, pp, 1-9. [3] M.P. Manhing and R.C. Reid, ASME Pap. 75-ENAs-22, Am. Sot. Mech. Engineers, 1975. [4] M. Lee, J. Lee and C. Chang, J. Chem. Eng. Jyn., 23 (1990) 130. [5] R.G. Copperthwaite, P.R. Davis, M.A. Morris, M.W. Roberts and R.A. Ryder, Calal. Lett., 1 (1988) 11. [6] T. Kodama, K. Tominaga, M. Tabata, T. Yoshida and Y. Tamaura, J. Am. Ceratn. SOL, 75 (1992) 1287. [7] CL. Zhang, T.H. Wu, H.M. Yang, Y.Z. Jiang and S.Y. Peng, Chem. J. Chin. Univ., 16 (1995) 955. [8] Joint Committee on Powder Diffraction Standards, Swarthmore, PA, 1989, ICPDS Card 19-629. [9] T. Katsura and Y. Tamaura, Blrll. Chem. Sot. Jpn., 5.2 (1979) [lo] ? Volenik, M. Seberiai and J. Neid, CXC/I. J. Phys., B25(1975) 1063. [ 111 H. Topsoe, J.A. Dumesic and M. Boudart, J. Phys. Paris, C6 (1974) 4ii. [ 121 R. Dieckmann, Ber. Bunsenges. Phys, C/rem., 86 (1982) 112. [ 131 L.S. Darken and R.W. Gurry, J. Am. Chem. SOL, 68 (1945) 798. [14] Y. Tamaura and M. Tobata, Nature, 346 (1990) 255.