Synthesis of highly loaded Cu/Ce mesoporous silica. Active catalyst for the simultaneous reduction of SO2 and NO with CO

Article abstract of DOI:10.1039/b516450h

We report the synthesis of a thermally stable, highly loaded (15 wt.%) Cu/Ce bimetallic mesoporous silica with high surface area and metal dispersion, which was the first template assisted mesoporous network successfully tested for the simultaneous reduction of SO₂ and NO with CO, achieving complete conversion of the reactants to elemental sulfur and N₂ at high space velocity 32 000 h^-1. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b516450h

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Synthesis of highly loaded Cu/Ce mesoporous silica. Active catalyst
for the simultaneous reduction of SO2 and NO with CO
Constantinos C. Pantazis* and Philippos J. Pomonis
Received (in Cambridge, UK) 21st November 2005, Accepted 24th January 2006
First published as an Advance Article on the web 7th February 2006
DOI: 10.1039/b516450h
So far template assisted mesoporous inorganic frameworks have
not yet been employed in deSOx operations and especially in the
simultaneous reduction of SO2 and NO with CO. This could be
due to the lack of suitable surface properties, induced by the
particular metal cations, necessary to activate both SO2 and NO
molecules, which compete for adsorption sites on the catalyst
surface. Moreover, high metal load and dispersion are also
important in order for high reaction rates to be achieved.
Recently, as a follow up of previous work with pure hexagonal
and Pm3n cubic mesoporous silica,22,23 we reported the synthesis
of highly loaded copper containing hexagonal mesoporous silica
using poly(acrylic acid) (Pac)–CTAB complexes as templates,
where Pac acts also as a carrier of the metal species on the
framework through complexation,24 providing materials with high
metal load and dispersion. The material exhibited excellent
performance for the catalytic reduction of NO by CO. However,
it proved unable to catalyze the simultaneous reduction of SO2 and
NO by CO.
We report the synthesis of a thermally stable, highly loaded
(15 wt.%) Cu/Ce bimetallic mesoporous silica with high surface
area and metal dispersion, which was the first template assisted
mesoporous network successfully tested for the simultaneous
reduction of SO2 and NO with CO, achieving complete
conversion of the reactants to elemental sulfur and N2 at high
space velocity 32 000 h21
.
The synthesis of thermally stable template-assisted mesoporous
silica is highly desirable as such inorganic frameworks are very
promising supports for heterogeneous catalysts on which a high
amount of catalytically active sites can be dispersed. However, in
most cases incorporation of various metal species at amounts
exceeding 6–7 wt.%1–10 leads to dramatic loss of mesoporosity and
surface area as a result of sintering effects.
Copper and ceria containing mesoporous silica have not been
widely studied probably due to the deterioration of the
mesoporous framework caused by the large size of cerium cations
especially at a high degree of incorporation.
Thus, based on the relevant literature, this work aims first at the
synthesis of a thermally stable and highly loaded Cu and Ce
containing mesoporous silica. The synthesis protocol is mentioned
in detail elsewhere.24 In the present case we have added 1.6 mmol
Cu(NO3)2?3H2O and 0.7 mmol Ce(NO3)2?6H2O (Merck) in the
reactant mixture so as Si/(Cu + Ce) = 8.7 (10.3 mol% Cu+Ce). The
total molar composition of the mixture was TEOS/C16TAB/
Pac2(2000 a.u.)/(Ce + Cu) = 1/0.35/0.013/0.12. The material
designated as Pac2C16CuCe was calcined in air at 600 uC for 5 h.
Then, a preliminary test on the simultaneous reduction of SO2
and NO with CO was conducted in order to assess the catalytic
activity of the material in the specific reaction.
By applying the ion exchange method Long et al.,11 synthesized
Al-MCM-41 that contained ,2% mol Ce with respect to silica and
specific surface area 904 m2 g21. They also developed a silicate
system containing Cu/Ce at 4/0.5% mol and specific surface area
870 m2 g21
.
Moreover, ceria containing MSU-X alumina has been synthe-
sized by direct incorporation of the Ce source in the reactant
mixture that resulted in a 5% mol Ce/MSU alumina possessing
specific surface area 357 m2 g21 after calcination at 600 uC.12 Also
MCM-41 type mesoporous silica containing 2% mol Ce has been
developed with a wormlike structure and specific surface area
845 m2 g21 after calcination at 550 uC.13
The reaction 2NO + SO2 + 4CO A 4CO2 + S + N2 has attracted
considerable interest as it results in the simultaneous elimination of
all three major air pollutants, i.e. NO, SO2 and CO. The difficulty
of this process is that a suitable catalyst has to overcome the
deactivation of NO reduction due to strong SO2 adsorption on its
surface.
Nitrogen adsorption measurements were performed at 77 K on
a Sorptomatic 9000 Fisons instrument after outgassing for 12 h at
473 K. X-ray diffraction measurements were acquired on a Bruker
Advance D8 system using Cu-Ka radiation (l = 1.5418 s) and a
step of 0.01u. Scanning Electron Microscopy (SEM) and Energy
Dispersive Spectrometric analysis (EDS) was performed on a Jeol
JSM 5600 at 20 kV equipped with an Oxford EDS X-ray
microanalysis apparatus.
This fact excludes noble metal based materials from potential
catalysts for the above reaction as they adsorb strongly SO2 and
suffer early deactivation.
In Fig. 1 the X-ray diffractograms of the calcined (b) and
uncalcined (a) Pac2C16CuCe samples both at the low and the
higher angle ranges are shown. A single strong reflection in the
range 1.5 , 2h , 4u is observed for the uncalcined (a) sample with
a d-spacing d100 = 4.58 nm. This mesophase is probably hexagonal
as this is the parent phase of the Pac2C16 system.22–24 The rest of
the higher order reflections of low intensity appear quite broad
indicating a disordered mesophase. Calcination at 600 uC for 5 h
results in a wormlike mesoporous network with a d100-spacing
Copper and cerium on various substrates are among the metals
14–18
19,20
and deSOx
of choice for deNOx
catalytic operations and
for the demanding operation of the simultaneous reduction of SO2
and NO.21
University of Ioannina, Department of Chemistry, 45110, Ioannina,
Greece. E-mail: me00596@cc.uoi.gr; Fax: +32651098795;
Tel: +32651098703
1268 | Chem. Commun., 2006, 1268–1270
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Fig. 2 Typical EDS spectra of the sample microanalysis along with a
SEM image of the analyzed region. (The star (*) corresponds to Au used
for the sample coating).
In Fig. 3, the N2 adsorption–desorption isotherms at 77 K for
the calcined sample are shown. The material exhibits a large BET
specific surface area 829 m2 g21. The characteristic condensation
step at P/Po 0.3–0.4 indicates a mesoporous material, with a pore
diameter of 3.4 nm as calculated by the Horvath–Kawazoe
method. However, the low height of the condensation step
indicates at a relevantly low mesopore volume and mesopore
population, while the initial, high adsorption step could imply the
presence of microporosity. A parallel hysteris loop is observed at
the desorption step, which ends before the condensation step at the
mesopores. This should be attributed to a secondary intraparticle
porosity formed by the sintering of the particles.25
Fig. 1 X-ray diffractograms of Pac2C16CuCe sample (a) uncalcined and
(b) calcined at 600 uC for 5 h both at low and high angle range (inset). The
stability of the wormlike mesoporous network is evidenced by the
appearance of the d100 reflection of the calcined sample at the same value
as the corresponding one of the uncalcined. The high dispersion of the
metal species is indicated by the lack of characteristic metal oxide
reflections at the high angle diffractogram (inset b).
Finally, in Fig. 4, the conversion profiles of a preliminary test on
the simultaneous catalytic reduction of SO2 and NO by CO are
shown. The catalytic test was performed in a lab scale plug flow
reactor similar to that described elsewhere26 at a steady state mode.
In the present experiment 200 mg of the solid Pac2C16CuCe
catalyst was used. The total flow rate of the reactant mixture (CO/
NO/SO2/He, 1.04%/0.22%/0.25%/98.49) was 200 ml min21, which
corresponded to a GHSV of 32 000 h21. The effluents were
analysed on a Carlo Erba GC using a combination of Porapac Q
and Molecular Sieve 13X columns.
value at 4.62 nm, which surprisingly remains practically unaffected
by the thermal treatment up to 600 uC. This result indicates no
shrinkage of the mesoporous network upon calcination and
implies the high thermal stability of the latter. We should note that
˚
the pure silica material undergoes a shrinkage of about 7 A upon
calcination (from d100 = 3.9 nm to d100 = 3.2 nm). Addition of the
metal species, although it degrades ordering and periodicity, seems
to lead to pore expansion and strengthening of the network, a fact
which indicates a favorable interaction with the latter.
In Fig. 2 a typical EDS microanalysis spectra of the calcined
sample is shown. The analysis indicates at an exceptionally high
concentration of the metal species that reaches the total of
11.6 mol% and specifically 8.1 mol% for Cu and 3.5 mol% for Ce
with respect to Si atoms or 15 wt.% total metal load if calculated
by considering formation of SiO2, CuO and CeO2 upon
calcination. We should note that analysis were performed in
several regions and magnifications of the material with practically
similar results, indicating the homogeneity of the latter.
Moreover, in spite of the high metal content, the dispersion of
the metal species into the inorganic framework remains high after
calcination, as indicated by the high angle diffractogram of the
calcined sample (Fig. 1, inset b), which shows no characteristic
reflections of the corresponding metal oxides even at this elevated
metal content. This result also implies a strong interaction of the
metal species with the matrix that should inhibit the formation of
large crystallites of the metal oxides maintaining the high
dispersion of the latter after calcination.
Fig. 3 N2 adsorption–desorption isotherms at 77 K and the correspond-
ing pore size distribution calculated according to the Horvath–Kawazoe
method for the calcined at 600 uC Pac2C16CuCe sample.
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mesoporous catalytic system to have been successfully tested for
the simultaneous reduction of SO2 and NO by CO towards
elemental sulfur and N2 respectively. Further investigation towards
a more complete view of the catalytic properties of the material in
the subject reaction is being undertaken.
We thank the European Union for funding this work under the
INORGPORE program (project G5RD-CT-2000-00317). We also
acknowledge
financial
support
from
the
projects
HERAKLEITOS and PYTHAGORAS from the EU and the
Ministry of Education (EPEAEK). We finally, acknowledge
assistance from the SEM and XRD units of the Network of
Laboratory Units and Centres of the University of Ioannina.
Notes and references
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Fig. 4 Percent remaining SO2, NO and CO in the products and % yields
of CO2, COS, N2 and N2O for the reaction indicated. Catalyst mass
200 mg at 32 000 h21
.
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The CO is in excess with respect to the stoichiometry of the
following reactions, that are believed to take place20 and cannot be
totally consumed.
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2CO + 2S A 2COS
(1)
(2)
(3)
(4)
(5)
2COS + SO2 A 2CO2 + 3S
2CO + 2NO A N2 + 2CO2
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byproducts COS and N2O along the whole tested temperature
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(2) in order that COS is not to be detected in large amounts.
Activity starts around 350 uC and 100% conversion of NO to N2 is
achieved at 500 uC. Complete conversion of SO2 to elemental
sulfur takes place between 550–600 uC.
The catalytic testing lasted about 8 h indicating no signs of
deactivation. We should note that the material was active at high
space velocity 32 000 h21 contrary to some relevant reports,27
which apply an SV in the range 2 500–10 000 h21 for the same
reaction.
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In conclusion, we report the synthesis of copper and cerium
bimetallic mesoporous silica, where the metal species are favorably
embodied in the network reinforcing it at the same time. The
material exhibits exceptionally high metal load, surface area,
thermal stability and metal dispersion compared to similar
systems. This material was the first template assisted bimetallic
1270 | Chem. Commun., 2006, 1268–1270
This journal is ß The Royal Society of Chemistry 2006