Pillared montmorillonites modified with silver: Temperature programmed desorption studies

Article abstract of DOI:10.1023/B:JTAN.0000033194.99509.c5

Alumina, zirconia and titania pillared montmorillonites additionally modified with silver were tested as catalysts of NO reduction with NH ₃ or C₂H₄. Ammonia was much more effective reducer of NO than ethylene. The silver

Full text of DOI:10.1023/B:JTAN.0000033194.99509.c5

Journal of Thermal Analysis and Calorimetry, Vol. 77 (2004) 115–123
PILLARED MONTMORILLONITES MODIFIED WITH
SILVER
Temperature programmed desorption studies
L. Chmielarz*, M. Zbroja, P. Kuœtrowski, B. Dudek,
A. Rafalska-£asocha and R. Dziembaj
Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Cracow, Poland
Abstract
Alumina, zirconia and titania pillared montmorillonites additionally modified with silver were
tested as catalysts of NO reduction with NH3 or C2H4. Ammonia was much more effective reducer
of NO than ethylene. The silver containing TiO2-pillared clay has been found to be the most active
catalyst for NO reduction both with NH3 or C2H4. Oxidation of the reducing agents by oxygen lim-
ited the NO conversion in the high temperature region. The ammonia and nitric oxide adsorption
sites were studied by the temperature programmed desorption methods (TPD).
Keywords: ammonia, DeNOx, ethylene, pillared montmorillonites, TPD
Introduction
Montmorillonite is a natural clay characterized by a laminar structure in which the
negative charge of layers is compensated by interlayer cations (e.g. Na+, Ca2+) [1, 2].
Pillared clays are obtained by replacement of these common cations by large
polynuclear cationic species, which during thermal treatment are transformed into
metal oxide pillars. Such modification of montmorillonite increases its microporosity
and surface area. The texture parameters can be controlled by the choice of a suitable
pillaring agent. The density and strength of acidic surface centers might be increased
by the acid-pretreatment of the clay materials, while the redox centers can be gener-
ated through the introduction of transition metals into the montmorillonite structure.
The pillared clays are thermally stable up to about 400–500°C. Due to such proper-
ties, montmorillonites intercalated with metal oxides were found to be very
interesting materials for catalytic applications [3–6].
Pillared clays modified with different transition metals were recognized as very
promising catalysts for NO reduction due to their high activity and stability in the pres-
ence of typical components of flue gases (e.g. SO2, water vapor) [7, 8]. This paper is
concerned with the study of the alumina, zirconia and titania pillared montmorillonites
additionally modified with silver as catalysts of NO reduction by ammonia or ethylene.
*
Author for correspondence: E-mail: chmielar@chemia.uj.edu.pl
1388–6150/2004/ $ 20.00
Akadémiai Kiadó, Budapest
© 2004 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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CHMIELARZ et al.: PILLARED MONTMORILLONITES
Experimental
The montmorillonite used in this study was the sodium-exchanged Wyoming benton-
ite (A-15). The cation exchange capacity of this clay is 83 meq per 100 g (the total
charge of interlayer cations which compensates negative charge of the montmorillo-
nite layers related to 100 grams of a dry clay material).
Alumina pillars were introduced to the montmorillonite by the ion-exchange
method using an aluminium hydroxy-oligomeric solution. The pillaring solution was
obtained by adding 0.1 mol NaOH into an AlCl3 solution until the molar ratio of
OH/Al=2.5 was reached. The modified montmorillonite was separated, washed with
distillated water to remove chloride ions and then dried (120°C/12 h).
The solution of the titania oligocations was prepared by adding TiCl4 into HCl
until the final concentrations of Ti4+ and HCl equal to 0.82 and 0.11 mol, respectively
were reached. The pillaring agent was then added to the montmorillonite suspension
until the Ti/clay ratio reached a value of 10 mmol g–1. The final product was sepa-
rated, washed and dried at 120°C for 12 h.
The montmorillonite was pillared with zirconia using an aqueous solution of zir-
conyl chloride (10 mmol Zr/1 g of clay). The modified clay was separated, washed
with distilled water to remove Cl– ions and then dried (120°C/12 h). A more detailed
description of the preparation procedure of the pillared montmorillonites was pre-
sented in a previous paper [9].
Silver was introduced into the pillared clays by the ion exchange method. One
gram of the pillared montmorillonite was added to 100 mL of the 0.02 mol silver nitrate
solution. The vigorously stirred suspension of the pillared clays was left in contact with
the solution of transition metal nitrate at 60°C for 12 h. The silver modified clays were
separated, washed and dried at 120°C for 12 h and then calcined at 400°C for 12 h.
The chemical composition of the samples was determined by X-ray fluores-
cence (XRF) using an Oxford 2000 instrument. The analytic response was calibrated
using standard materials. The X-ray diffraction studies of the calcined
montmorillonites were performed with a PW3710 Philips X’pert diffractometer us-
ing CuKα radiation (λ=0.154178 nm). The BET measurements were performed using
an ASAP 2010 instrument (Micromeritics). Prior to N2 adsorption at liquid nitrogen
temperature (–196°C), the samples of calcined clay were outgassed at 350°C under
vacuum for 12 h.
The temperature-programmed desorptions of NH3 (NH3-TPD) or NO (NO-TPD)
were carried out in the temperature range of 70–600°C in a fix bed continuous flow
microreactor. The desorption temperature was measured with a K-type thermocouple lo-
cated in a quartz capillary immersed in the catalyst bed. The molecules desorbing from
the samples were monitored on-line by a quadrupole mass spectrometer (VG Quartz)
connected to the reactor outlet via a heated line. Prior to TPD experiments, the clay sam-
ple (50 mg) was outgassed at 400°C for 1 h in a flow of helium (20 ml min–1). Subse-
quently, the sample was cooled down to 70°C and saturated for about 30 min in a flow of
1% NH3/He or 1% NO/He (20 ml min–1). Then the catalyst was purged in a helium
stream until a constant baseline was attained. Desorption was carried out with a linear
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CHMIELARZ et al.: PILLARED MONTMORILLONITES
117
heating rate (β=10°C min–1) in a flow of He (20 ml min–1). Calibration of the
quadrupole mass spectrometer with commercial mixtures was performed to recalcu-
late the detector signal into a desorption rate.
The activity of the modified clays as catalysts of NO reduction by ammonia was
studied in a fixed-bed flow reactor. The reactant concentrations in the outlet stream
were continuously measured using a quadrupole mass spectrometer (VG Quartz)
connected on-line to the reactor. Prior to the reaction, each sample (200 mg, particle
diameter 125–180 µm) was outgassed in pure helium at 400°C for 30 min. The com-
position of the gas mixture at the reactor inlet was [NO]=[NH3]=0.25%, [O2]=2.5%.
Helium was used as a carrier gas at a total flow rate of 40 ml min–1. The intensities of
the mass lines corresponding to all reactants and products were measured at a given
temperature at least for 30 min after the reaction had reached a steady-state. The sig-
nal of the helium line served as an internal standard to compensate possible small
fluctuations of the operating pressure. The sensitivity factors of analysed lines were
calibrated using commercial mixtures of the gases. The possible changes in the molar
flow caused by NO conversion were negligible in the diluted reaction mixtures.
A similar experimental system was used for testing the clay materials as catalysts
of NO reduction with ethylene. However, in this case a gas chromatograph (Varian
3400 SX) was used for the outlet gases detection. GC was equipped with two packed
columns (Porapak Q and Molecular Sieves 5A) and a TCD detector. Prior to the cata-
lytic test the samples (200 mg, particle diameter 125–180 µm) were outgassed
(400°C/30 min) in a flow of helium. The reactant mixture of 0.25% NO, 0.25% C2H4,
2.5% O2 and 97% He entered to the reactor with a total flow rate of 40 ml min–1.
Results and discussion
Table 1 presents the chemical composition and surface areas of the modified
montmorillonites. Apart from the metals listed in this table, water and small amounts
(below 1 wt. %) of Na2O, K2O, MgO were also detected. The modification of the pil-
lared clays with AgNO3 solution resulted in introduction of a very large amount of
silver. The transition metal content was significantly higher than its quantity neces-
sary for the compensation of negative charge of the montmorillonite layers. Thus, it
seems that silver is deposited in the form of oxide clusters, which are thermally re-
duced during calcination.
Figure 1 presents the XRD patterns recorded for the raw montmorillonite and its
modifications. Position of the (001) peak, which allows to determine the interlayer
distance, for the clays pillared with alumina and zirconia was shifted to lower values
of 2θ angle. The obtained results show that pillaring of montmorillonite with Al2O3
resulted in an increase of the interlayer distance from 0.32 to 0.96 nm, while in the
case of the ZrO2-intercalated clay the interlayer spacing increased to 1.19 nm. A dis-
appearance of the (001) peak after pillaring of the parent montmorillonite with TiO2
suggests that the obtained material is characterized by a delaminated structure (non
parallel ordering of the clay layers). The broad peak at about 26° can be attributed to
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CHMIELARZ et al.: PILLARED MONTMORILLONITES
Table 1. The chemical composition and surface areas of modified montmorillonites
Composition/mass %
Sample
S
BET/ m2 g–1
Al2O3
SiO2
TiO2
ZrO2
Fe2O3
Ag2O
Mt-Na
16.84
23.25
13.25
6.17
63.50
58.51
33.34
28.32
19.52
47.20
24.03
0.73
0.47
0.00
0.00
10.42
6.72
3.81
1.29
0.86
4.90
2.47
0.00
0.00
38
Mt-Al
254
151
279
210
242
156
Mt-Al-Ag
Mt-Ti
0.11
0.00
38.61
0.00
51.79
35.73
0.16
0.00
Mt-Ti-Ag
Mt-Zr
4.27
0.00
26.55
0.00
8.30
31.25
15.95
Mt-Zr-Ag
4.26
0.09
44.82
Fig. 1 X-ray diffraction patterns of montmorillonite and its modifications. Q-quartz,
C-crystobalite, A-anatase, Ag-metallic silver
the presence of anatase (A), which was formed by a thermal decomposition of the
pillaring agent deposited on the montmorillonite surface.
Modification of the pillared clays with Ag+ cations results in an appearance of a
new peak at about 38°, which is assigned to the presence of metallic silver. During
the ion-exchange procedure, silver was deposited in the clay material in the form of
separate cations and metal oxide clusters, which were thermally reduced to metallic
silver (Ag) during the calcination process. A disappearance of the (001) peak, ob-
served in the case of the Mt-Zr-Ag sample, suggests that silver clusters are localized
partially in the interlayer space. Probably, these metal particles, which have various
sizes, disturb ordering of montmorillonite layers.
The maxima at 26° and 28°, distinctly seen in the diffractogram of the parent
clay, are attributed to the presence of the crystobalite (C) and quartz (Q) impurities.
The intercalation of the parent montmorillonite with metal oxide pillars resulted
in a significant increase in its surface area nearly by an order of magnitude (Table 1)
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CHMIELARZ et al.: PILLARED MONTMORILLONITES
119
mainly due to the increase of microporosity of the pillared materials. The
modification of the pillared clays with silver lowered their surface area by about 40%
for Mt-Al, by 25% for Mt-Ti and by 35% for Mt-Zr. This might be explained by the
blocking of interlayers with silver clusters.
The pillared clays modified with silver were tested as catalysts for reduction of
NO by ethylene or ammonia. Figure 2 shows the results of catalytic studies per-
formed with C2H4 as a reductant over the pillared montmorillonites doped with sil-
ver. Nitrogen was the only detected N-containing product. For all the studied cata-
lysts, the reaction begins at a temperature of about 100°C. In the case of the
Mt-Al-Ag and Mt-Zr-Ag samples, the NO conversion slowly increases to 10–14% at
about 250°C. At higher temperatures, very small changes in the reaction rate were
only detected. The Mt-Ti-Ag clay has been found to be much more active. Below
250°C, the NO conversion measured over this catalyst was very similar to those de-
tected for the Mt-Al-Ag and Mt-Zr-Ag samples. However, in the range of 250–400°C
a sharp increase in the NO conversion up to 60% was observed. For all the studied
catalysts, the ethylene conversion was significantly higher than the conversion of
NO, especially at higher temperatures. Thus, it should be concluded that ethylene
does not react selectively with NO but is also oxidized by oxygen present in the inlet
stream. This undesired process dominates at higher temperatures. It should be no-
ticed that the NO conversion by ethylene over the pillared clays non-modified with
silver did not exceed 5% in the whole studied temperature range (not shown).
The results of catalytic tests performed with NH3 as a reductant of NO are pre-
sented in Fig. 3. N2 and H2O are desired products, while N2O is an undesired one.
Fig. 2 Results of catalytic tests obtained for modified montmorillonites in reduction of
NO by ethylene
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CHMIELARZ et al.: PILLARED MONTMORILLONITES
Fig. 3 Results of catalytic tests obtained for modified montmorillonites in reduction of
NO by ammonia
Formation of any other gases was not detected. The modification of the pillared clays
with silver drastically increased their catalytic activity. The results of catalytic tests
performed over the pillared montmorillonites non-modified with transition metals
were presented in our previous paper [9]. The obtained results indicate clearly that
ammonia is much more effective reducing agent than ethylene. Among the studied
samples, the Mt-Ti-Ag catalyst was found to be the most active one. The NO conver-
sion that occurred over this catalyst exceeded 90% in the range of 280–400°C. At
higher temperatures, the reaction rate decreased due to the competitive oxidation of
ammonia by oxygen. The NO conversion over Mt-Al-Ag begins at about 150°C and
increases to 70% at 470°C. In the low temperature region (T<300°C) the Mt-Zr-Ag
sample was found to be the least active. However, at higher temperatures its activity
significantly exceeded that measured for the Mt-Al-Ag sample. For all the studied
catalysts, the selectivity to nitrogen is very high and does not drop below 95%.
The temperature programmed desorptions of NH3 (NH3-TPD) and NO
(NO-TPD) have been performed in order to recognize the reaction mechanism.
Figure 4 shows the results of NH3-TPD experiments. Ammonia desorption pattern re-
corded for the Mt-Al-Ag sample is spread in the range of 140–550°C and consists of
at least two unresolved maxima centered at about 220 and 400°C. The shape of the
desorption spectra is typical for NH3-TPD patterns of the alumina pillared montmo-
rillonite [10]. Thus, it seems that chemisorption of ammonia takes place mainly on
the surface of montmorillonite layers and alumina clusters. Introduction of silver into
the titania pillared montmorillonite produced a new kind of surface acidic centers,
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Fig. 4 NH3-TPD profiles of alumina, titania and zirconia pillared clays modified with silver
that are represented by the sharp ammonia desorption peak at about 320°C. Addition-
ally, modification of the TiO2-pillared clay with Ag+ cations decreased amount of
strongly chemisorbed ammonia molecules that desorb above 400°C [10]. The ammo-
nia desorption pattern recorded for the Mt-Zr-Ag catalyst is spread in the range of
130–540°C. Modification of the zirconia pillared clay with silver limited the amount
of weak acidic centers, that corresponds to desorption of ammonia in the low temper-
ature region of 150–300°C [11].
Results of NO-TPD measurements are presented in Fig. 5. The NO desorption
pattern recorded for the Mt-Al-Ag catalyst consists of two maxima centered at about
150 and 370°C (Fig. 5a). It is supposed that nitric oxide chemisorbs on the two types
of sites present on the surface of Mt-Al-Ag. Similar NO-TPD patterns were obtained
for the alumina pillared montmorillonite that was not modified with transition metals
[10]. So, it seems that NO chemisorption sites are localized mainly on the clay layers
or alumina clusters. Analogous results were obtained for the Mt-Zr-Ag catalyst (Fig.
5c). In this case, the intensity of NO desorption was however significantly higher and
formation of N2O in the range of 325–370°C was detected. Evolution of oxygen was
not observed. Probably, oxygen that was formed during conversion of NO into N2O
oxidized metallic silver to silver oxide. Desorption of nitric oxide from the Mt-Ti-Ag
sample takes place in the range of 120–360°C with the maximum centered at about
240°C. Chemisorbed NO was partially decomposed to nitrogen and oxygen in the
temperature range of 150–370°C. However, the evolution of N2 in this temperature
region was only detected. It is most likely that oxygen directly oxidizes silver to sil-
ver oxide. The NO desorption pattern does not contain peaks typical for TiO2-pil-
lared montmorillonites [8]. Thus, the majority of the surface in the Mt-Ti-Ag sample
is covered with silver.
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CHMIELARZ et al.: PILLARED MONTMORILLONITES
Fig. 5 NO-TPD profiles of alumina – a, titania – b, and zirconia – c pillared clays
modified with silver
Conclusions
Large amount of silver introduced into the pillared clays by the ion-exchange method
suggests that Ag exists on the montmorillonite surface of the form of metal oxide
clusters rather than separate cations. Such silver oxide clusters are reduced to metal-
lic silver during calcination of the samples at 400°C in the static air atmosphere.
The titania pillared clay modified with silver (Mt-Ti-Ag) was the most active cata-
lyst for NO reduction by ethylene. Silver introduced into the zirconia or alumina interca-
lated montmorillonites activates them considerably less effectively. Significantly higher
amounts of NO were converted by ammonia. The Mt-Ti-Ag clay was found to be the
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CHMIELARZ et al.: PILLARED MONTMORILLONITES
123
most active catalyst also for this process. In the presence of this catalyst, the NO conver-
sion exceeded 90% in the temperature range of 270–400°C. At higher temperatures, the
rate of NO reduction decreased due to the oxidation of ammonia by oxygen. Such an ef-
fect was not observed for the Mt-Al-Ag and Mt-Zr-Ag samples. However, the activity of
these catalysts was lower than that of the Mt-Ti-Ag one. For all the clay catalysts the se-
lectivity to nitrogen did not drop below 95% in the studied temperature range.
The TPD studies have shown that NO as well as NH3 chemisorption sites exist
on the surface of the modified clays. The ammonia desorption spectra obtained for
the Mt-Al-Ag and Mt-Zr-Ag samples are very similar to those recorded for the alu-
mina and zirconia pillared clays, respectively. Thus, the chemisorption of NH3 mole-
cules takes place mainly on the montmorillonites layers or pillars surface. In the case
of the Mt-Ti-Ag sample, apart from maxima characteristic for the titania pillared
clay, a new ammonia desorption peak, caused by the presence of silver, appeared.
At least two types of NO chemisorption sites on the surface of the Mt-Al-Ag and
Mt-Zr-Ag samples were detected. The NO-TPD spectra recorded for these catalysts
are very similar to those obtained for the alumina or zirconia intercalated clays. Thus,
the NO chemisorption takes place mainly on the pillared montmorillonites surface.
Totally different results were obtained for the Mt-Ti-Ag sample. In this case, a disap-
pearance of maxima characteristic for the TiO2 pillared clay and formation of new
one, caused by the silver presence, was observed.
The new type of NO and NH3 chemisorption sites that are present on surface of
the Mt-Ti-Ag sample might be responsible for a high activity of this catalyst in the
NO reduction processes.
* * *
This work was supported by KBN (Committee for Scientific Research, Poland) under contract No.
4T09B 06522.
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