Low temperature CO oxidation over iron-containing MCM-41 catalysts

Article abstract of DOI:10.1039/b416852f

Different MCM-41 samples containing framework iron were prepared and tested in CO oxidation showing unprecedented high activities after reduction in hydrogen above 773 K. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b416852f

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Low temperature CO oxidation over iron-containing MCM-41 catalysts
a
Agnes Szegedi, Mih a´ ly Heged u˝ s, J o´ zsef L. Margitfalvi* and Imre Kiricsi
a
a
b
´
Received (in Cambridge, UK) 4th November 2004, Accepted 21st December 2004
First published as an Advance Article on the web 26th January 2005
DOI: 10.1039/b416852f
Different MCM-41 samples containing framework iron were
prepared and tested in CO oxidation showing unprecedented
high activities after reduction in hydrogen above 773 K.
MCM-41 materials with spherical and irregular morphology
13,14
were synthesised as described earlier.
Further details are given
in the notes.{ Characteristic features of samples prepared are given
in Table 1.
Low temperature CO oxidation has a great academic and applied
CO oxidation has been studied in the temperature range 297–
1
interest. Supported platinum, tin–platinum, and gold containing
2
3
673 K using Temperature Programmed Oxidation (TPO)
4
catalysts were studied in this reaction. Hopcalite (CuMn
2 4
O ) and
technique.{ Details of TPO and FTIR techniques used can be
found elsewhere.
5
9,15
Co
3
O
4
based catalysts are the most active noble-metal-free
catalysts used in CO oxidation. Iron containing catalysts, such
6
as MFI type ferrisilicates, show high activity only above 673 K. A
Fig. 1 shows the influence of different thermal treatment
procedures on the activity of a Fe-MCM-41 catalyst. The
treatment in oxygen at 623 K gave rise to very low activity.
Further increase of the temperature of calcination had no influence
on the activity. Omitting the calcination step and reducing the
recent study indicated that nanosized iron oxide is highly active
7
above 523 K. Recently, some attempts were also made using
combinatorial approaches to find new catalyst compositions
8
containing no noble metals. Mixed oxides with general formula
sample at 673 K in H resulted in a more active catalyst. However,
2
Al
1
Mn6.7Co93.3
O
x
showed the highest activity. Consequently, there
this catalyst had an unstable behaviour having both increasing and
is a great interest to develop noble metal free catalysts for low
decreasing parts in the TPO curves. On the other hand the
reduction at 773 or 873 K provided a highly active and stable
temperature CO oxidation.
9
10
2
Based on results of Sn–Pt/SiO and Au/MgO catalysts we
have proposed a new mechanism for CO activation with the
1
1
involvement of ‘‘metal ion–metal nano-cluster’’ ensemble sites.
4+
Over Sn–Pt/SiO
2
catalysts in situ formation of surface Sn , while
d+
on Au/MgO catalysts ionic forms of gold (Au ), were detected by
9,12
different spectroscopic methods.
According to this concept a
metal ion stabilized in the neighbourhood of a supported metal
nanocluster interacts with the lone pair of the oxygen atom of a
chemisorbed CO molecule. The result of this interaction is the
perturbation of the CO molecule leading to its high reactivity even
at low temperatures.
The aim of this study is to demonstrate that ‘‘metal ion–metal
nanocluster’’ ensemble sites formed in a metal ion environment can
oxidize CO at relatively low temperature.
Fig. 1 The influence of the pretreatment procedures on the catalytic
2
properties of Fe-MCM-41(20)(A). 1—oxygen at 623 K; 2—H at 673 K;
*joemarg@chemres.hu
3—H
2
at 773 K; 4—H
2
at 873 K.
Table 1 X-ray and surface characteristics of Fe-MCM-41 samples
a
21h
b
SA /m g
2
21
c
PD /nm
d
WTH /nm
Samples
Si/Fe
Fe cont./mmol g
d100/nm
Fe-MCM-41(45)
Fe-MCM-41(20)(A)
Fe-MCM-41(10)
44.9
18.0
10.0
23.6
20.1
38.1
22.8
0.360
0.860
1.410
0.667
0.778
0.421
0.688
3.78
3.84
3.92
3.47
3.80
727
847
770
1004
1400
1049
107
2.47
2.27
2.28
2.31
2.36
2.30
—
1.90
2.16
2.24
1.70
1.72
1.53
—
Fe-MCM-41(20)(B)
Fe-MCM-41(20)(C)
Fe-MCM-41(38) (imp)
e
3.32
f
g
2
Fe–SiO (spherical)
a
13
Sample (A) was prepared by the sol–gel method of Unger et al., sample (B) by applying the same procedure with modifying the
composition of the synthesis gel, and sample (C), having irregular morphology with hydrothermal synthesis.
1
3 b
c
Specific surface area. Mean
d
pore diameter calculated by BJH method. Wall thickness, WTH 5 2/3
1/2
e
100 – PD. Iron was introduced by impregnation. The same
f
d
g
h
synthesis procedure like in case of Fe-MCM-41(20)(A), but without using template. X-ray amorphous. Related to 1g sample calcined at
273 K.
1
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catalyst with T50 , 323 K. This high activity is unprecedented in
CO oxidation using iron-containing catalysts.
. Fe-MCM-41(20) (T50 5 300 K). Consequently, these results
indicate that there is an optimum in the iron content in Fe-MCM-
41 type catalysts.
The observation that high activity appears only after reduction
above 773 K strongly suggests that in Fe-MCM-41 new surface
species with metallic character have been formed.
In the next set of experiments the following parameters were
varied: (i) surface area of Fe-MCM-41, (ii) spherical vs. non-
spherical particles, (iii) form of iron, i.e. ‘framework’ vs. surface
iron. As emerges from Fig. 3 both Fe-MCM-41(imp) and Fe–
Fig. 2 shows the influence of dehydroxylation on the TPO
pattern. As emerges from Fig. 2 after reduction at 523 K the initial
conversion was below 10% and the T50 value was around 573 K.
However, the addition of a dehydroxylation step at 773 K in He
increased considerably the activity resulting in T50 5 338 K.
Results given in Fig. 2 also show that the reduction temperature
has more pronounced influence on the activity than the
temperature of dehydroxylation.
SiO (spherical) have much lower activity than Fe-MCM-
2
41(20)(A). This finding indicates that the nano-environment of
iron containing sites has a great influence on the catalytic activity.
Only iron stabilised in MCM-41 structure resulted in highly active
surface species for low temperature CO oxidation. In addition, the
results show also that the higher the surface area of spherical Fe-
MCM-41(20) the higher the catalytic activity (compare samples
(A) and (B)).
The increase of the reduction temperature from 523 to 673 K
followed by treatment in He at 773 K decreased further the T50
value to 323 K. The highest activity was obtained after reduction
Methods of in situ infrared spectroscopy were used to prove the
partial reduction of iron in Fe-MCM-41 type catalysts. Fig. 4
shows the FTIR spectra of chemisorbed CO at RT on Fe-MCM-
2
in H at 773 K followed by treatment in He at the same
temperature resulting in about 65% conversion at room tempera-
ture and 100% conversion at 338 K.
41(20)(A) after reduction at 773 K. The build up of surface species
The catalyst reduced at 773 K is highly active, its initial activity
at 300 K is higher than 50%. However, for the catalysts treated in
He omitting the reduction step has much lower activity.
Consequently, the high temperature reduction is a crucial step in
the process leading to a highly active Fe-containing catalyst.
Results shown in Fig. 2 indicate that this process can be further
enhanced by high temperature treatment in an inert atmosphere.
Further studies will be required to elucidate the character of
interactions during high temperature treatment in He. It is
tentatively suggested that in addition to dehydoxylation partial
has a strong pressure dependence. At low CO pressure (pCO
5
2
0 mbar) there are only two IR bands: an intensive broad
21
asymmetric band at 2170 cm
and a weak band around
21
080 cm . The broad band has a shoulder around 2165 cm .
21
2
Upon increasing the CO partial pressure to 50 and 100 mbar
2
1
additional bands appear at 2048 and 2128 cm . Further increase
in pCO resulted in no additional IR bands.
21
The band around 2048 cm
unambiguously indicates the
and Fe/SiO catalysts the
band around 2050 cm was assigned as CO adsorbed on reduced
formation of metallic iron. In Fe/Al
2
O
3
2
21
3+
auto-reduction of Fe can take place in samples treated only in He
at high temperature.
16
21
iron. It has to be emphasized that the band at 2048 cm was not
observed on Fe-MCM-41(20) reduced at 573 K. In ref. 11 upon
The results strongly indicate that the activity of Fe-MCM-41
catalysts depends on both the extent of reduction of iron and the
amount of surface OH groups in the neighbourhood of the active
sites. The former is controlled by the temperature of reduction,
while the latter by the temperature of thermal treatment
extending the reduction temperature to 873 K the intensity of the
21
band at 2050 cm
became more pronounced. There are
21
additional references confirming that the band around 2050 cm
17
can be assigned to linear CO adsorbed on metallic iron. It is
assumed that the particle size of the iron nano-clusters formed
after high temperature reduction is very small as no metallic iron
was detected by powder XRD.
2
procedures performed either in H or He. It is suggested that
high surface concentration of OH hinders the access of reactants
and oxygen to iron containing active sites.
The exposure of the Fe-MCM-41 sample to CO + O
2
mixture
In the next series the Si/Fe ratio in Fe-MCM-41 was varied in
the range 10–50. The T50 values decreased in the following order:
Fe-MCM-41(10) (T50 5 320 K) . Fe-MCM-41(45) (T50 5 313 K)
resulted in pronounced alteration in the FTIR spectra as shown in
21
Fig. 4/D. Due to the presence of oxygen the band at 2048 cm
Fig. 3 Influence of different structural variables on the catalytic activity:
Fig. 2 The influence of the temperature of dehydroxylation (catalyst: Fe-
2
1—Fe–SiO (spherical); 2—Fe-MCM-41(imp); 3—Fe-MCM-41(20)(C);
MCM-41(20)(B)). 1—H : 523 K; 2—H
2
2
: 523 K, He: 773 K; 3—H : 673 K;
2
4—Fe-MCM-41(18)(A); 5—Fe-MCM-41(24)(B). (For abbreviations see
Table 1.)
4
—H : 673 K, He: 773 K; 5—H
2
2
773 K; 6—He 773 K.
1
442 | Chem. Commun., 2005, 1441–1443
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Notes and references
1
3
{
Experimental: Modifying the procedure of Unger et al. , C16TMABr,
water, ethanol and TEOS were mixed in the desired proportion.
Fe(NO .9H O (Fluka) was added and stirred for 30 min. Conc. ammonia
3
)
3
2
was poured into this solution causing immediate gel precipitation. In a
second synthesis procedure the composition of the synthesis gel was
modified, in consequence of using diluted ammonia. This second procedure
resulted in better ordered Fe-MCM-41 samples with higher surface area.
2
The template removal was carried out at 770 K in N and air. Further
1
4
details on the synthesis can be found elsewhere. Fe-MCM-41(20)(imp)
was prepared by impregnation of 2 g Si-MCM-41 with methanolic solution
21
3 3 2
of dried Fe(NO ) (80 g l ), stirring for 24 h under N atmosphere
followed by filtering, washing with methanol, drying and calcining at 373 K
9
and 773 K, respectively. Catalytic tests: TPO technique has been used. The
2
1
amount of catalyst: 0.100 g, total flow rate 70 ml min , PCO 5 16 Torr,
PO2 5 16 Torr, heating rate 10 K min . Catalysts characterisation and
21
catalytic tests were performed in a multipurpose, ASDI RXM 100
equipment (Advanced Scientific Designs, Inc.). Before testing, the
Fig. 4 FTIR spectra of adsorbed CO at RT on Fe-MCM-41(20)(A).
A—pCO 5 20 mbar; B—pCO 5 50 mbar; C—pCO 5 100 mbar;
D—pCO 5 100 mbar, pO2 5 50 mbar.
catalysts were reduced in H
was used for product analysis. The temperature required to achieve
0% conversion (T50) was used to compare the activity of catalysts.
2
and/or dehydroxylated in He. MS method
5
2
1
entirely disappeared and a new band appeared at 2195 cm . The
FTIR studies: FTIR measurements. In situ infrared spectra were recorded
at room temperature using Nicolet Impact 400 FTIR instrument. In
3+
letter has been assigned to CO adsorbed on Fe sites.
21
21
1
8
the spectral region 4000–1100 cm the resolution was 1 cm . The
description of the cell and the high vacuum apparatus used can be found
15
elsewhere.
Based on literature data the following further assignments
2+
2
1
were made: band around 2080 cm : CO adsorbed on Fe in the
2
1
silicate matrix; band around 2128 cm : fingerprint of iron ions
with a mean oxidation number between 2 and 3; band around
2+
1
A. Bourane and D. J. Bianchi, J. Catal., 2004, 222, 499.
2
1
3+
2
170 cm : Fe cations stabilised in the neighbourhood of Fe .
2 A. N. Akin, G. Kilaz, A. I. Isli and Z. I. Onsan, Chem. Eng. Sci., 2001,
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The results of the activity test indicate that iron modified MCM-
4
1 is highly active in low temperature CO oxidation. FTIR results
showed unambiguously the partial reduction of iron in hydrogen
above 773 K.
3
G. C. Bond and D. T. Thompson, Catal. Rev.—Sci. Eng., 1999, 41, 319;
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Based on the results obtained in this study we propose that there
are at least four structural elements involved in the activity control:
4 G. M. Schwab and G. Drikos, Z. Phys. Chem., 1940, 185, 405.
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¨
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6
channels, (ii) the extent of reduction of iron, (iii) a proper balance
between ionic and metallic forms of iron, and (iv) low surface
coverage of silanol groups.
4
68.
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2
9
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1
after high temperature treatment in H and He showed high
2
N. Bogdanchikova, Catal. Today, 2002, 72, 157; J. L. Margitfalvi,
´
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higher than the activity of iron-oxide containing catalysts
6
8
,7
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an exact proof that after high temperature reduction in H the
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Fe-MCM-41 sample contained both metallic and ionic forms of
iron. It was shown for the first time that supported iron
containing catalysts having both ionic and metallic forms of iron
are active in low temperature CO oxidation. The results provided
further proof that ‘‘metal ion–metal nanocluster’’ ensemble sites
have a crucial role in CO activation.
1
´
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Partial financial support by OTKA (grant T 043570) is greatly
acknowledged.
1
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Imre Kiricsi
Institute of Surface Chemistry and Catalysis, Chemical Research
a
Center, Hungarian Academy of Sciences, 1525, Budapest, POB 17,
Hungary. E-mail: joemarg@chemres.hu; Fax: 36-1-325-7554;
Tel: 36-1-325-7747
2
9.
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Department of Applied and Environmental Chemistry, University of
Szeged, Rerrich B e´ la t e´ r 1., 6723, Szeged, Hungary.
E-mail: kiricsi@chem.u-szeged.hu; Fax: 36-62 544 619;
Tel: 36-62 544 619
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 1441–1443 | 1443