In situ XRD and in situ IR spectroscopic analyses of structural change of goethite in methane oxidation

Article abstract of DOI:10.1006/jssc.2000.8992

This paper examines the methane oxidation activity of iron oxides from goethite synthesized from different types of iron compounds. The variation of crystal structure caused by heat treatment of goethite under methane and oxygen gas mixtures was investigated by in situ X-ray diffractometry and in situ infrared spectroscopy. Methane oxidation commenced at a low temperature of 573 K when goethite was used as a catalyst, after the goethite had changed into hematite by dehydration at proximately 493 K. It was found that the active species in methane oxidation was hematite.

Full text of DOI:10.1006/jssc.2000.8992

Journal of Solid State Chemistry 156, 225}229 (2001)
doi:10.1006/jssc.2000.8992, available online at http://www.idealibrary.com on
In Situ XRD and In Situ IR Spectroscopic Analyses of Structural Change
of Goethite in Methane Oxidation
Masahiro Katoh,* Masanao Orihara,- Toshihiro Moriga,* Ichiro Nakabayashi,*ꢁꢀ
Shigeru Sugiyama,* and Suminori Tanaka-
*
Department of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima, 2-1 Minami-Josanjima, Tokushima 770-8506; and
Okura Industrial Co., Ltd., 1515 Nakatsu, Marugame, Kagawa 763-8508, Japan
-
Received April 17, 2000; in revised form September 14, 2000; accepted October 6, 2000; published online December 21, 2000
that reduction of unburned granular carbon particles, together
This paper examines the methane oxidation activity of iron with the accomplishment of complete combustion in the
oxides from goethite synthesized from di4erent types of iron incinerators, makes it possible to restrict dioxin generation.
compounds. The variation of crystal structure caused by heat
treatment of goethite under methane and oxygen gas mixtures
was investigated by in situ X-ray di4ractometry and in situ
infrared spectroscopy. Methane oxidation commenced at a low
temperature of 573 K when goethite was used as a catalyst, after
the goethite had changed into hematite by dehydration at ap-
proximately 493 K. It was found that the active species in meth-
ane oxidation was hematite. ( 2001 Academic Press
This study examines the catalytic activity of iron oxide
obtained from goethite synthesized from di!erent types of
iron compounds. As a model experiment, an investigation
was made with the oxidation of methane, which is the least
reactive of the hydrocarbons. Structural change caused by
heat treatment of goethite in the gas mixture of methane and
oxygen was followed by in situ X-ray di!ractometry and in
situ infrared spectroscopy.
Key Words: goethite; hematite; methane oxidation; in situ
XRD; in situ IR.
MATERIALS
INTRODUCTION
As starting substances, three types of goethite were used
(4). Type 401 was obtained by air-oxidizing an alkali-sus-
The catalytic activity of iron oxides in the oxidation of pension liquid brought about by mixing FeSO ) 7H O
ꢃ
ꢂ
H , CO, CH , and ole"ns is never vigorous enough in aqueous solution with Na CO in N . Type 402 was ob-
ꢂ
ꢃ
ꢂ
ꢄ
ꢂ
comparison with precious metals, transition metals such as tained by dispersing the 401 into distilled water with the
nickel, cobalt, etc., and the oxides of the metals. However, addition of Na O ) nSiO and H PO as a sintering retar-
ꢂ
ꢂ
ꢄ
ꢃ
iron oxides, which are widely available in a natural state, are dant after dehydration at 392 K. Type 403 was obtained by
inexpensive and seldom a!ect human beings. For this rea- air-oxidizing an alkali suspension liquid brought about by
son, the oxides are widely used as combustion catalysts. For mixing FeSO ) 7H O and NaOH. From TEM photographs
ꢃ
ꢂ
example, a report has been released suggestions that addi- shown in Fig. 1, we can see that the three types of goethite
tion of the oxides to incinerators helps restrict dioxin, car- are all spindle- or needle-shaped. The needle-shaped crystals
bon monoxide, and nitrogen oxides in the exhaust fumes of the goethite are characteristic of those published in the
(
1). Thus garbage bags or resin bags for daily procurement literatures (5}9). It is also found that the particle diameter of
into which goethite (a-FeOOH) is impregnated are in 401 and 402 is approximately 0.25 lm, which is approxim-
production (2).
ately ꢀ of that of 403. The surface areas of the three speci-
ꢄ
Dioxin discharged from incineration plants is produced mens were measured with a conventional BET nitrogen
by reactions of unburned granular carbon particles, reacting adsorption apparatus (Shibata P-700). The BET surface
with dioxin precursors through de novo synthesis with ma- areas of 401, 402, and 403 pretreated at 473 K were 82, 74,
terials such as inorganic chlorine, air, and moisture (3). It is and 18 mꢂ/g, respectively.
also produced by decomposition or synthetic reactions of
chlorobenzenes or chlorophenols. Accordingly it is believed
MEASUREMENTS
The catalytic experiments were performed in a "xed-bed
continuous-#ow quartz reactor operated at atmospheric
ꢀ
To whom correspondence should be addressed. Fax: #81-88-655-
7
025. E-mail: ichiro@chem.tokushima-u.ac.jp.
2
25
0
022-4596/01 $35.00
Copyright ( 2001 by Academic Press
All rights of reproduction in any form reserved.

2
26
KATOH ET AL.
GC-8APT) equipped with a TC detector and integrator
Shimadzu C-R6A). TG}DTA was done by using a
Shimadzu DTG-40 Thermoanalyzer with a heating rate of
0 K/min from room temperature to 1073 K under air#ow
10 ml/min). Powder X-ray di!raction data were collected
using high-performance di!ractometer (Rigaku
(
1
(
a
RINT200VHF>) with monochromatized CuKa radiation
with a source power of 40 kV}150 mA for phase identi"ca-
tion of the specimens and for in situ measurement in meth-
ane/oxygen mixed gas at higher temperature. The sample
was mounted on a platinum holder. The same gas mixture
as that used in the catalytic experiments was introduced to
the reaction chamber at a rate of 50 ml/min. The heating
rate to the given temperatures was 10 K/min, and the tem-
perature was held for 1 min prior to the in situ measurement.
The scanning rates were 23/min for phase identi"cation and
1
03/min for the in situ measurement, respectively. Transmis-
sion infrared spectra of the samples under reaction were
recorded on an FT-IR spectrometer (Shimadzu 8200PC)
with 4 cm\ꢀ resolution. The sample pellets were prepared
by pressing powdered 401 mixed with or without KBr as
a diluent. Details of the sample holder for the IR measure-
ments are described elsewhere (11). The KRS-5 window was
used for high transmittance in the low wave number region.
RESULTS AND DISCUSSION
Catalytic Activity
It was found that no methane oxidation occurred even at
8
73 K in the absence of the catalyst. The reaction was
started at as low temperature as 573 K on goethite (12).
Under the present conditions, carbon dioxide and water
exclusively were produced. This result that neither carbon
monoxide nor C compounds (such as ethane) were found is
ꢂ
consistent with the previous papers (6, 7). Figure 2 shows
the conversion of methane (C ) and oxygen (C ) on
+
-
goethite. The C and C were calculated by the following
formulas, respectively,
+
-
CH (P)
C (%)" 1!
ꢃ
;100,
[1]
[2]
+
G
CH (P)#CO (P)
H
FIG. 1. TEM photographs of the three types of goethite: (A) 401,
B) 402, and (C) 403.
ꢃ
ꢂ
(
O (P)
C (%)" 1! ꢂ
;100
ꢀ
O (R)
G H
ꢂ
pressure. Details of the reactor design have been described
elsewhere (10). Prior to reaction, the catalyst was calcined in where O (R) represents the mole number of oxygen in the
ꢂ
situ in a helium #ow (F"25 ml/min) at a given reaction gas introduced, and CH (P), CO (P), and O (P) represent
ꢃ
ꢂ
ꢂ
temperature for 20 min. A half-gram of the catalyst was the mole numbers of methane, carbon dioxide, and oxygen
loaded (="0.5 g). The gas used was diluted with helium so in the product stream. The carbon mass balances were
that the pressures of methane and oxygen would be kept at 101$5%, based on our 98 runs. The conversions of both
3
.4 and 20.3 kPa, respectively (P(CH )"3.4 kPa and methane and oxygen, for which catalytic activities were in
ꢃ
P(O )"20.3 kPa). The reactants and products were ana- the order 401'403'402, increased with increasing reac-
ꢂ
lyzed with an on-stream gas chromatograph (Shimadzu tion temperature. With 401, a decrease was seen in the rate

STRUCTURAL CHANGE OF GOETHITE IN METHANE OXIDATION
227
from 473 to 523 K, as shown in Fig. 3. This reaction temper-
ature determined by TGA is in good agreement with that
reported in the literature (8). Little change was seen up to
1
073 K. High-temperature XRD measurement in air was
performed for 401, as seen in Fig. 4. All the di!raction peaks
at 473 K could be assigned to goethite, whereas those at
5
03 K could be assigned to hematite (a-Fe O ). Di!raction
ꢂ ꢄ
pro"les at 483 and 493 K showed mixed phases of these
materials. These results show that structural change accom-
panied by dehydration from goethite to hematite occurs
from 473 to 503 K. From the fact that the goethite becomes
catalytically active above 573 K, it is suggested that the
active species of the reaction is hematite. It is notable that in
Fig. 4, a pronounced re#ection width anisotropy is apparent
in the XRD pattern of hematite. It has been reported that
the (012), (014), (024), and (214) peaks of hematite are prefer-
FIG. 2. CH and O conversion as a function of temperature over the
three types of goethite: 401 (E), 402 ()), and 403 (᭛). Conditions:
ꢃ
ꢂ
=
with He.
"0.5 g, F"25 ml/min, P(CH )"3.4 kPa, P(O )"20.3 kPa diluted
ꢃ ꢂ
of the increment of the conversion above 773 K, exhibited as entially broadened with respect to the others (5, 9).
an S shape in the graph. This might derive from contact To examine the structure variation of the hematite in the
between methane and oxygen with the catalyst becoming course of methane oxidation, in situ high-temperature XRD
restricted due to the decrease in the partial pressure of the measurements were conducted for the catalysts in the gas
methane caused by the consumption of a large amount of mixture, as shown in Fig. 5. Above 753 K, di!raction peaks
methane. Especially, 401 showed a conversion ratio as high assigned to magnetite appeared beside those assigned to
as nearly 80% at 723 K. The larger number of covered hematite and increased in intensity as the temperature was
active sites for methane conversion with substances and/or raised. When the reaction temperature exceeded 753 K,
reactants would lead to the lower number of attacks to fresh methane oxidation occurred, resulting in a shortage of oxy-
active sites by substrates. This causes the decrease of the gen in the system. The hematite would be unable to fully
increment of the conversion.
supplement the shortage of oxygen with the oxygen con-
The particle diameter, obtained from TEM photography, tained in the gas, so that hematite partially changed into
of 401 (0.25 lm) was smaller than that of 403 (0.7&0.8 lm). magnetite. The temperature when the magnetite appeared
The BET surface area of 401 is larger than that for 403 (82 was in a good agreement with that at which there was
and 18 mꢂ/g, respectively), which results in higher conver- a decrease in the rate of the increment of the methane
sion. Comparing 402 (particle diameter 0.25 lm, speci"c conversion.
surface area 74 mꢂ/g) with 401, we see that the methane
conversion was extremely small, despite the fact that their
particle diameters and speci"c surface areas were almost the
In Situ IR Analysis
same. This would suggest that the sintering-restricting
In situ IR measurement was performed using goethite
agents, Na O ) nSiO and H PO (the existence of P on the (401) pretreated at room temperature in vacuo
ꢂ
ꢂ
ꢄ
ꢃ
4
02 surface was con"rmed by XPS), covered the goethite (1.0;10\ꢂ Pa) for 2 h in an in situ IR cell. The spectra in
particles in 402. It should be noted that the speci"c activity Fig. 6 were obtained in methane and oxygen mixed gas. At
i.e., surface-area normalized conversion of both methane room temperature, in addition to bands assigned to the
(
and oxygen) was in the order 403'401'402. It is gener-
ally accepted that the activity for the oxidation of methane
is strongly in#uenced by Na-containing species on the cata-
lyst surface (13), indicating that the nature of the Na-con-
taining species on 403 may be rather di!erent from that on
4
01 and 402. In the following, we describe in situ high-
temperature XRD and in situ IR analyses of 401, on which
the oxidation activity of the methane was the highest and
the sintering e!ects seemed to be negligible.
In Situ High-Temperature XRD Analysis
TG}DTA measurement for goethite in air showed that an
endothermic reaction accompanied by weight loss occurred heating rate"10 K/min, under air#ow.
FIG. 3. TG}DTA curves of goethite (401). Conditions: ="&0.06 g,

2
28
KATOH ET AL.
FIG. 4. High-temperature XRD pro"les for goethite (401) in air at
various temperatures. Conditions: ="&0.5 g, heating rate"10 K/min,
holding duration prior to the measurement"1 min, under methane/
oxygen mixed gas #ow diluted by He.
gaseous methane (about 3020 and 1300 cm\ꢀ) (14), free
water (about 3700 and 1600 cm\ꢀ) (14), and carbon dioxide
(
about 2350 cm\ꢀ) (14) in air which exists in the circumfer-
ence of the cell, bands assigned to the goethite (about 900,
00, 600 and 450 cm\ꢀ) (5, 15) were observed. The broad
FIG. 6. In situ infrared spectra of goethite (401) diluted with KBr in
methane and oxygen mixed gas at various temperatures.
8
band (about 3200 cm\ꢀ) assigned to the OH groups of
goethite (a-FeOOH) was also observed. Little change in the
spectrum was seen from room temperature to 513 K. From bands for water increased, the proposal of a dehydration
13 to 533 K, however, there were considerable changes. reaction from goethite to hematite is supported. Structural
5
The bands assigned to goethite disappeared and were re- change from goethite to hematite at 513}533 K was con-
placed by bands assigned to the hematite (550 and "rmed with IR measurement in the gas mixture of methane
4
50 cm\ꢀ) (5, 15). In particular, since the bands assigned to and oxygen, which was almost in coincidence with the
the OH groups disappeared while the intensities of the high-temperature XRD measurement. Since little change of
the band was seen from 533 to 673 K, it is con"rmed that
there was no e!ect of the IR emission that is generated with
an increase in the sample temperature (16) on the spectrum
in the measured temperature region.
However, no change was observed in the spectrum corre-
sponding to the oxidation of methane, which is believed to
occur at temperatures above 573 K. This might result from
the use of KBr as a diluent, preventing the iron catalyst from
coming into contact with the methane. With such a situ-
ation in mind, pure 401 (6.3 mg) was molded into a pellet
having a diameter of 13 mm and thickness of 57 lm. The
pellet was pretreated at 623 K in vacuo (4.0;10\ꢄ Pa) for
1
0 h and the goethite was converted to hematite, in order to
con"rm the activity of hematite to methane oxidation. For
the self-supported pellet, an excellent IR spectrum was ob-
tained over 4000}800 cm\ꢀ although the spectrum was
saturated at a wave number below 800 cm\ꢀ. The results of
the measurements are shown in Fig. 7. From room temper-
ature to 573 K, only the bands assigned to gaseous methane,
free water, and carbon dioxide in air were observed. The
di!erences between the intensities of the methane peaks
obtained at all measurement temperatures could not be
recognized, because of the small catalyst quantity in the IR
FIG. 5. In situ high-temperature XRD pro"les for hematite in methane
and oxygen mixed gas at various temperatures.

STRUCTURAL CHANGE OF GOETHITE IN METHANE OXIDATION
229
The results of in situ X-ray di!raction measurements and
in situ IR absorption measurements show that in methane
oxidation, the goethite is converted to hematite and is
occasionally changed into magnetite, depending on the con-
ditions. It can be deduced that the active catalyst in methane
oxidation is hematite.
REFERENCES
1
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M. Ukita, in &&Proceedings of the 9th Annual Conference of the Japan
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3
. B. K. Gullett, K. R. Bruce, and L. O. Beach, Chemosphere 20, 1945
(1990).
4
5
. T. Nakamura and H. Kurokawa, Phys. Status Solidi A 155, 207 (1996).
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6
. A. C. S. Brown, J. S. J. Hargreaves, and B. Rijniersce, Catal. ¸ett. 53,
7
(1998).
FIG. 7. In situ infrared spectra of hematite in methane and oxygen
mixed gas at various temperatures.
7. A. C. S. Brown, J. S. J. Hargreaves, and B. Rijniersce, Catal. ¹oday 45,
47 (1998).
8
9
. L. A. Pe
J. Mater. Chem. 9, 1839 (1999).
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H rez-Maqueda, J. M. Criado, C. Real, J. S[ ubrt, and J. Boh aH cek,
cell. However, the band assigned to carbon dioxide adsor-
bed (17) on hematite clearly appeared at 673 K, due to high
molar absorption coe$cient of carbon dioxide. It was con-
sidered that the carbon dioxide adsorbed was generated by
(2000).
1
0. S. Sugiyama, Y. Iguchi, H. Nishioka, T. Minami, T. Moriga, H.
Hayashi, and J. B. Mo!at, J. Catal. 176, 25 (1998).
methane oxidation. It can be also explained by the IR 11. T. Yamazaki, I. Watanuki, S. Ozawa, and Y. Ogino, ¸angmuir 4,
4
33 (1988).
measurement showing that methane oxidation did progress.
Thus the structural change from geothite to hematite at
1
1
2. S. Sugiyama, T. Minami, T. Moriga, H. Hayashi, and J. B. Mo!at,
J. Solid State Chem. 135, 86 (1998).
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Sanger, Catal. Sci. Eng. 32, 163 (1990).
5
13}533 K was con"rmed in Fig. 6 and methane oxidation
was con"rmed at 673 K in Fig. 7. These results show that
the active catalyst in methane oxidation was hematite.
14. G. Herzberg, &&Infrared and Raman Spectra of Polyatomic Molecules.''
Van Nostrand, New York, 1945.
1
1
5. A. Raman, B. Kuban, and A. Razvan, Corros. Sci. 32, 1295 (1991).
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(1993).
CONCLUSIONS
This study has shown that 401 goethite is the most e!ec-
tive combustion catalyst among the three types of goethite.
1