Adsorption and low-temperature oxidation of CO over iron oxides

Article abstract of DOI:10.1016/j.molcata.2006.02.051

Different iron oxides, usually used as gold supports, were tested as catalysts for CO oxidation at low temperature. Calcinations of synthesized magnetite at different temperatures, before the activity test, caused its chemical transformations but differences in the activity were not noticeable in all the cases. A sample calcined at 873 K was significantly less active than samples calcined at 473 and 673 K. FTIR measurements in a vacuum showed that the differences in the activity were not caused by the chemical composition but by amount of surface -OH groups.

Full text of DOI:10.1016/j.molcata.2006.02.051

Journal of Molecular Catalysis A: Chemical 252 (2006) 103–106
Adsorption and low-temperature oxidation of CO over iron oxides
a,∗
b
c
ˇ
Goran Smit , Stanka Zrn cˇ evi c´ , K a´ roly L a´ z a´ r
a
Department of Chemistry, Faculty of Philosophy, J.J. Strossmayer University of Osijek, Lorenza J a¨ gera 9, Osijek 31 000, Croatia
b
Faculty of Chemical Engineering and Technology, University of Zagreb, Savska 16, Zagreb 10 000, Croatia
c
Institute of Isotope and Surface Chemistry, P.O. Box 77, CRC H-1525, Budapest, Hungary
Received 15 December 2005; accepted 12 February 2006
Available online 27 March 2006
Abstract
Different iron oxides, usually used as gold supports, were tested as catalysts for CO oxidation at low temperature. Calcinations of synthesized
magnetite at different temperatures, before the activity test, caused its chemical transformations but differences in the activity were not noticeable
in all the cases. A sample calcined at 873 K was significantly less active than samples calcined at 473 and 673 K. FTIR measurements in a vacuum
showed that the differences in the activity were not caused by the chemical composition but by amount of surface −OH groups.
©
2006 Elsevier B.V. All rights reserved.
Keywords: CO oxidation; Iron oxides; −OH groups; 57Fe M o¨ ssbauer; FTIR
1
. Introduction
2. Experimental
Roles of different iron oxides as gold supports in CO oxi-
2.1. Catalyst preparation
dation were doubtful so far. Namely, opposite opinions about
influences of magnetite (Fe3O4), maghemite (␥-Fe2O3) and
haematite (␣-Fe2O3) on the mentioned reaction were published.
During pre-treatment, there is always some possibility for trans-
formations of those oxides to each other but reported results
about their influences on the activity were contradicted. One
of the most active catalysts for CO oxidation was prepared
by co-precipitation when gold was supported on haematite
A starting material was magnetite [7] prepared by adding
a stoichiometric mixture of Fe(II)-(FeSO4·7H2O, Kemika) and
Fe(III)-ions (FeCl3·6H2O, Riedel-de Ha e¨ n) at a ratio 1:2 into
0.7 M NH4OH containing a citric acid trisodium salt. After vig-
orously stirring (1500 rpm) for 30 min at room temperature and
pH 9.79, the solid was separated and washed twice, dried at
323 K, ground and heated in air at 433 K for 4 h.
[
1]. Formation of magnetite [2] and maghemite [3] after pre-
treatment in hydrogen decreased the activity of Au/Fe2O3 cat-
alysts. But in some articles, appearance of maghemite with
haematite [4] or pure maghemite [5,6] as gold supports was
attributed for increased activity. In this paper, the catalytic activ-
ity of those iron oxides for low-temperature oxidation of CO
was investigated as a part of a project about their roles in gold
catalysis.
2.2. Characterization
57Fe M o¨ ssbauer spectra were obtained at ambient tempera-
ture and at 77 K by conventional KFKI spectrometer operated in
a constant acceleration mode. The reported isomer shift values
were related to metallic ␣-iron and the accuracy of the positional
−
1
data was ± 0.03 mm s . The characteristic data were extracted
from the spectra presuming they were composed of Lorentzian
shaped peaks.
2.3. FTIR measurements
∗
FTIR experiments were performed in a standard vacuum line
(pressure of 1 × 10 Pa) with a quartz infrared cell contained
Corresponding author. Fax: +385 31 212 514.
E-mail address: gsmit@ffos.hr (G. Smit).
−
3
ˇ
1
381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.02.051

ˇ
G. Smit et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 103–106
1
04
CaF2 windows and an external furnace section. Catalyst sam-
ples were prepared as 16 mm diameter self-supporting discs of
Table 1
5
7Fe M o¨ ssbauer parameters of M-200, M-400 and M-600 catalysts recorded at
300 K
0
.03 g by pressing (2 tonnes) the powder between two polished
steel dies. Before the experiments, the samples were calcined
Sample Composition IS
QS
(mm s 1) (mm s
MHF LW
(T)
RI
(%)
−
−1
−1
(mm s )
3
−1
)
in situ by flowing air (100 cm min ) for 2 h at 473 K (code:
M-200), 673 K (code: M-400) and 873 K (code: M-600). After
calcination, the cell was evacuated for 30 min with the disc at the
same temperature and then cooled to room temperature. Three
spectra were obtained: before (label “start” in the figures) and
after introducing a pulse of CO under pressure of 3999 Pa and
after evacuation of the cell for 10 min. In general, 100 scans were
recorded at 298 K using a Perkin-Elmer Model 1710 FTIR spec-
M-200 FeO (spm)
Maghemite
0.35
0.37
0.73
–
–
42.4
0.54
1.36
87a
13
27a
54
19
M-400 FeO (spm)
␣-Fe2O3(
0.32
0.37
0.37
1.05
0.22
0.21
–
50.9
48.5
1.26
0.29
0.55
1)
Fe2O3
(
(
1)
2)
M-600 ␣-Fe O
2
3
0.37
0.37
0.20
0.21
51.5
50.7
0.23
0.33
61
39
␣-Fe2O3
−
1
trometer fitted with an MCT detector and operating at 4 cm
resolution.
IS: isomer shift related to metallic ␣-iron; QS: quadrupole splitting; MHF: mag-
netic hyperfine field; LW: line width (full width at half maximum); RI: relative
intensity (spectral contribution).
a
Sum of the doublet and the curved superparamagnetic background; spm:
2
.4. Catalytic measurements
superparamagnetic component.
The activity of iron oxides for CO oxidation was measured
in a fixed-bed reactor at atmospheric pressure using 0.1 g of
netic behaviour. A small portion of maghemite in that sample
can be proposed because of moderated magnetic field values.
An average particle size of 3 nm can be taken as a very rough
approximation and this sample is not recognizable by XRD. The
overwhelming parts of the M-400 spectra are characteristic for
haematite but at 300 K there is still visible a small contribution of
the superparamagnetic component. M-600 has typical haematite
spectra but the sample is not completely homogeneous (labels
3
−1
the catalysts. The O2 + He + CO (20/78/2 cm min ) mixture
passed through the catalyst bed with a space velocity (SV) of
3
−1
−1
h . The effluent gases were analyzed
CAT.)
about 60,000 cm g(
by quadrupole mass spectrometer (QMS 311, Balzers). Before
the catalytic activity tests, the samples were pre-treated by heat-
3
−1
ing in a stream of O2/He = 20/80 cm min for 2 h at 473 K
M-200), 673 K (M-400) and 873 K (M-600).
(
(1) and (2) in the tables) because slightly different MHF values
can be distinguished.
3
. Results and discussion
Figs. 2–4 represent FTIR spectra before and after adsorption
of CO on the iron oxides in a vacuum and after evacuation of
the IR cell.
Fig. 1 shows M o¨ ssbauer spectra of tested samples, while
Tables 1 and 2 contain their chemical compositions and 57Fe
M o¨ ssbauer parameters recorded at 300 and 77 K, respectively
After introduction of CO, M-200 shows increased bands at
−1
3
–
637 and ≈3400 cm which could be ascribed to H–bonded
[
8,9]. As can be seen, M-200 exhibits typical superparamag-
OH groups and adsorbed water, respectively [10]. Increase
−
1
of peaks at 2958, 2870 and 1563 cm , which originate from
adsorbed residual citrates [11] added during the synthesis,
proves formation of adsorbed formates, HCOO
[12,13].
ad)
(
−
1
(Increase of bands at 1377 and 1360 cm is not shown for the
−1
sake of brevity.) Appearance of a small band at 2364 cm , as
Table 2
5
7Fe M o¨ ssbauer parameters of M-200, M-400 and M-600 catalysts recorded at
77 K
Sample Composition IS
QS
mm s 1) (mm s
MHF
(T)
LW
−1
(mm s )
RI
(%)
−
−1
(
)
M-200 FeO (spm)
Maghemite
0.43
0.43
.42
0.93
–
–
–
50.5
47.0
1.15
0.51
0.75
74a
15
11
0
M-400 ␣-Fe2O3(1)
0.47
0.43
0.35
0.11
53.7
51.8
0.26
0.54
51
49
Fe2O3
M-600 ␣-Fe2O3(
1)
0.47
0.43
0.37
0.19
54.0
52.7
0.28
0.33
75
25
␣
-Fe2O3(
2)
IS: isomer shift related to metallic ␣-iron; QS: quadrupole splitting; MHF: mag-
netic hyperfine field; LW: line width (full width at half maximum); RI: relative
intensity (spectral contribution).
a
Fig. 1. 57Fe M o¨ ssbauer spectra of M-200, M-400 and M-600 catalysts recorded
at 300 and 77 K.
Sum of the doublet and the curved superparamagnetic background; spm:
superparamagnetic component.

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G. Smit et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 103–106
105
a result of CO2 adsorption [14], indicates partial oxidation of
very reactive formates what explains increased amount of water
as well [15]:
CO + –OH → HCOO(ad)
(1)
(2)
2HCOO(ad) + O(support) → 2CO2 + H2O
Water formed in this reaction can easily dissociate and keep a
constant amount of –OH groups on a surface of the oxide with
uncoordinated metal cations and oxide anions [16]:
n+1 2−
−
n+−
OH
M
O
+ H2O → HO M
(3)
This reaction is not so intensive on this sample because of
adsorbed citrates and that is the reason for more adsorbed water
and H-bonded –OH groups.
M-400, besides a band characteristic for H-bonded –OH
Fig. 2. FTIR spectra of M-200 catalyst recorded before and after adsorption of
CO and after evacuation of the IR cell.
−1
groups (3629 cm ), has a band of free or almost free –OH
−
1
groups at 3670 cm [12] because of less surface water in com-
parison with M-200. Also, on this sample there are no adsorbed
citrates which were removed by calcination at 673 K. Increase
−
1
of a band at 3480 cm is a consequence of adsorbed water pro-
duced during the oxidation of formates (Eq. (2)). Differently
from the sample M-200, appearance of the water on this sample
is not so obvious. Namely, dissociation (Eq. (3)) is more feasi-
ble because cations of iron are free of citrates but the signals for
surface –OH groups decreased: carbonate-type species (bands
−
1
at 1617, 1420 and 1223 cm ) were formed after adsorption of
CO2 on –OH groups [14,17].
In the same conditions, on M-600 there is only appearance of
−
1
doublet at 2143 cm , corresponds to gaseous CO [16] which
disappears after evacuation. It means there is no oxidation of CO
and it could be explained only by lack of adsorbed water and
–OH groups because the chemical compositions of M-400 and
M-600 are almost the same.
The catalytic behaviour of iron oxides M-200, M-400 and
M-600 are shown in Fig. 5. It is obvious that sample M-600 is
less active than the other two. On this oxide, the reaction starts
Fig. 3. FTIR spectra of M-400 catalyst recorded before and after adsorption of
CO and after evacuation of the IR cell.
Fig. 4. FTIR spectra of M-600 catalyst recorded before and after adsorption of
CO and after evacuation of the IR cell.
Fig. 5. Conversion of CO as a function of catalyst temperature.

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G. Smit et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 103–106
1
06
at 500 K, while samples M-200 and M-400 are active even at
40 K. Samples M-200 and M-400 show almost identical activ-
Acknowledgment
3
ity. (The reaction over M-200 was stopped at 473 K to prevent
possible transformation of magnetite and maghemite which usu-
ally occurs at this temperature.) It seems that the main path in
low-temperature CO oxidation over formates is by oxygen from
a gas phase because adsorbed citrates on M-200 did not influ-
ence on the activity. Complete conversion of CO over M-400 and
M-600 are practically achieved at the same temperature (583 K),
but sample M-400 is more active at lower temperature. As can
be seen from Fig. 5, 50% conversion over M-400 is achieved
at 515 K and over M-600 at 542 K, respectively. Those results
are in accordance with our preliminary results [18] with gold
dispersed at the same supports.
We are grateful to Dr. James A. Anderson at University of
Dundee for his kind permission to use the FTIR spectrometer.
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